It's a bit of a cliché, but it's true: The longer any given conversation in the United Kingdom (or Ireland), the higher the chance that the weather will be mentioned at some point.
Why the weather?
Because it's the only "safe" topic that you can discuss with a stranger. As Kate Fox described it in her book Watching the English, the British are chronically shy and protective of their personal space, and they use the weather as a conversational totem to avoid accidental invasion of said personal space. The most fundamental rule of "weatherspeak" is to agree with the first speaker, regardless of whether or not you actually agree with them (e.g. one might respond to "It's too hot today" with "Yeah, it is, but I don't mind it myself").
The main issue with talking about something other than the weather is that in Britain, everything is Serious Business to someone, and as they say, the British have two hobbies: complaining about the weather and arguing. The only way not to risk an argument is to talk about something everybody has had enough with at some point or another.
One reason for this might be that due to the climate and the way properties and infrastructure are constructed, it means that Britain only copes well within a fairly narrow range of temperatures, so that on any given day it's likely to be either too hot, too cold, or too wet. This of course gives plenty of reasons to complain about the weather and keep up that popular national past time.
Houses in Britain, unlike those towards the Mediterranean, are heavily insulated and designed to keep heat in, and very few residential properties have air conditioning, given that it would be a significant investment for something that will probably only be in use for a couple of weeks per year, and some years not even that. This means that during heatwaves, where the UK can have temperatures on a par with continental Europe, the population has to cope with high temperatures but without the infrastructure and tools other countries have to help manage this.
Conversely, Britain also copes badly with extended snow and ice, since temperatures are rarely low enough to result in prolonged exposure to it, and resulting in chaos to the transport infrastructure when it does occur. More northern countries have extensive winter driving experience, whereas few would bother with winter tyres in the UK, and the number of accidents in such weather tells you enough about how few drivers can cope well with the conditions.
So what is British weather?
It's always the same, and yet it's never the same, which again makes for perfect conversation fodder.
Britain tends to have three seasons: a cold season, a wet season, and a cold and wet season. John Cleese once told David Letterman, "We have a national holiday. It's called summer." And there's more truth in that than one might realise, because Britain is indeed such a rainy place that one would be lucky to get a whole week of uninterrupted sunshine.
The British Isles are actually quite far north — at the same latitude as Canada, Belarus, Russia and parts of Northern Europe — but it rarely sees extreme weather in either direction, because the Gulf Stream comes in over the Atlantic from the southwest, moderating the colder air from the north, so it never gets that cold. Commentators might point out that without the Gulf Stream, London should be as cold as Winnipeg.note The high latitude and the prominent ocean winds mean it never gets that hot either. But the Gulf Stream also brings rain and storms with it, hence all the rain.
Bill Bryson once noted seeing a forecast of "Warm and dry, with cooler and rainy spells", and commented that such a forecast could apply to any day of the year, and for all he knew it was printed every day. He further noted that unless you decide to go hiking up Ben Nevis in your dressing gown, you can probably get away with wearing the same thing all year round — as long as it comes with a hood or umbrella. In the words of the Prophet Connolly: "There is no bad weather, only bad clothing."
British summer is a bit hard to define; Brits consider anything over 14 degrees Celsius (57-58 Fahrenheit) quite pleasant, and anything over 20 degrees (68 Fahrenheit) pretty hot. In all seriousness, it rarely goes over 30 degrees (once or twice a year perhaps) and has never gone over 40 (the U.K. record is 38.5 degrees in Kent in 2003). The best weather in Britain is considered "cricket and strawberries" weather, and it's almost guaranteed that at least one British newspaper is going to have photographs of bikini-clad sunbathers. If it goes over 35 degrees, the yellower press will start issuing horror stories about heat stroke, UV rays, and how the entire country is going to melt or die of thirst in a Biblical-scale drought. (Australians find all this rather amusing, given that their country can get really hot.)
British winter also tends to be mild. Snow can happen, but it's uncommon, and any significant amount of snow (by which we mean about an inch and a half) in most parts of the country will grind it to a halt, particularly in the cities and on the railways, and the news coverage will report breathlessly on it. Heavy snow is somewhat more common in Scotland, but not by much. All of this, of course, does not prevent British Christmas cards, decorations, and illustrations from being full of pretty snow and snowmen. A phenomenon more indicative of the British relationship with snow can be seen in Doctor Who; whenever it snows, there's generally an alien involved, and they're usually up to no good.
Of course, this is where climate change rears its ugly head, as British seasons are getting slightly but noticeably more extreme each year. 2010 saw a snowfall so severe that schools were shut and motorways blocked for several consecutive days. This, of course, is of no import to older folk, who will almost always remember something similarly bad When I Was Your Age... (and the winters of 1947 and 1963 and 1979 and 1981-82 were all notoriously cold and snowy, so they might have a point).
One often overlooked aspect of British seasons is that the British Isles are quite far north, so the days tend to be very long in the summer and very short in the winter. The disparity is especially stark in Scotland, where in some parts one can even see the Northern Lights. The sun can set as late as 10pm in midsummer and as early as 3pm in deep winter. This has also led to some degree of Fantastic Religious Weirdness here on Earth, as Britain's substantial Muslim community is occasionally at a loss over how to treat Ramadan, which is based on the Muslim lunar calendar and migrates from season to season, meaning they might have to fast for 15 or even 20 hours if Ramadan falls during the summer. Most imams suggest to follow approximate Mecca time and fast from 6am to 6pm, but many will go for the entire daylight period, even in the summer, and even in Scotland (with typical Scottish stubbornness).
The rain is the universal constant to British weather. Virtually every day, somewhere in the country will have a downpour. In Wales, both technically and proverbially the dampest part of Britain, the weather forecast will predict rain on pretty much any given day. The East Coast is supposed to be drier, but that's just relative; they might get away with two or three days of sunshine before a thunderstorm finishes it off.
Rain very often factors into the British sporting culture, but in very different ways depending on the sport. Cricket will pause when it feels so much as a drizzle — perfect time for a tea break, but it's a wonder how the sport could have become so popular in a country so rainy. Wimbledon was once notorious for rain interrupting matches, before they eventually built a retractable roof over Centre Court. On the other hand, football will only stop if the pitch is literally frozen or flooded, and even then only in the professional ranks. Rugby won't be called off for anything less than a direct meteor strike (and then only for long enough to prop up the posts).
British people are perversely very proud of their rain. Glasgow celebrated being listed as the rainiest city in the country by average rainfall, which came as a shock to the inhabitants of Cardiff. And when a dude from Hawai'i posted on Facebook that he lived in the rainiest place on Earth, where it rained for 247 consecutive days, British readers bragged that their country was much rainier than that, with one commenter from North Wales calling the 247-day rainy stretch "a drought" where he was from.
Floods were common once upon a time, and still happen on occasion, especially in the Midlands and South West. However, they've largely been prevented through human intervention; the frequently flooded Fens in East Anglia were drained in the 19th and 20th centuries (there was even an attempt in the 17th century, which failed miserably), and London is protected from the sea by the Thames Barrier (i.e. that thing with three sails near City Airport, although that's more against very high tides).
Despite all this, British weather authorities will occasionally insist that there is a drought going on and enact a hosepipe ban. Brits tend to react to such pronouncements with incredulity and ignore them, with a few suggesting the Weather Agency might be doing this on purpose to tempt the Gods. (The real reason they do this is if there's too much sun for too long, the soil gets too hard to absorb any rain that does come, and this inhibits the replenishment of the country's aquifers — and thus its main water supply.)
Britain sees a lot of wind, of varying strengths and directions, such that an expert weatherspeaker will note the peculiar nature of the wind on that particular day. The British Isles facing the Atlantic Ocean, the wind likes to come in from that direction, but that part of Europe being particularly low-lying, you can get wind from pretty much everywhere. It's pervasive enough that a favourite pastime of schoolchildren on a windy day is to lift their jacket up over their heads and walk into the wind (this is much more fun than it sounds).
Britain usually doesn't see hurricanes, because the sea is too cold for that. There are occasionally storms with hurricane-force winds, but since hurricanes by definition occur in the tropics, such storms are technically not hurricanes, in spite of the fact that they can be quite dangerous and destructive to persons and property. This is why BBC weather forecaster Michael Fish was technically correct when he said, in advance of the Great Storm of 1987, that there wasn't a hurricane on the way. On a few very rare occasions, Britain gets hit by a storm that used to be a hurricane (in 1986, Hurricane Charley, after trashing North Carolina and Virginia, managed to cross the Atlantic and dump historic amounts of rain on the British Isles, causing millions of pounds in damage and killing eleven people). During a bad storm, there's usually someone (often in Cornwall) to prove the adage that there's a fine line between bravery and stupidity by trying to surf the big waves.
Britain has a surprisingly high number of tornadoes, with the highest number of reported tornadoes relative to land mass of any country in the world. However, such tornadoes pale by comparison, in both number and intensity, to the kind typically found in the United States.
Tropes like A Foggy Day in London Town may lead one to believe that Britain, and especially London, is incredibly foggy. In fact, it used to be like that, but not anymore, at least in part because such fog was really smog — it mixed with the huge amount of pollutants being belched into the atmosphere as Britain industrialised in the 19th and early 20th centuries. The Great Smog of 1952, which killed an estimated 12,000 people in greater London, was the impetus for some long-needed legislation clamping down on pollutants and mandating clean air across the country, so that's why you don't see that level of fog anymore.
Some places in Britain can still reliably get fog, though — narrow river valleys Oop North are prone to mist and fog in the winter, and thick haar or "sea fog" can be seen in more northerly coastal cities like Edinburgh — but it's basically no longer a London phenomenon, and it's not an everyday occurrence anywhere in the country anymore.
Other weird British weather phenomena
The only real constant to British weather is the persistent rain; everything else can vary wildly, and with modern climate being what it is, that variance is just getting wilder:
- April is often the month when the Sun is first seen each year, and this can cause hill fires — presumably, they're just not used to the sun.
- In 2012, a section of the North Midlands got hit by a drought and a flood at the same time. They called it "The Wettest Drought on Record".
- The North of England saw a week of blizzards and -13°C temperatures, followed immediately by a week of heatwaves and +32°C temperatures.
- The Calder Valley flooded four times in the span of three weeks.
- In 2012, The Midlands experienced hailstones in July.
- There's even the occasional earthquake, in spite of Britain not having been seismically active for hundreds of millions of years. They're naturally very small (don't talk to Californians or Japanese about them), but if one happens, expect it to take over the news for a few days.
In other words, there's a reason "The weather in London" used to be a meme on The Other Wiki.