Crowning Music of Awesome - The game uses the ULLAdubULLA remix album for its soundtrack, and certain tracks (such as The Fighting Machine) make for undeniably awesome battle music.
The Martian intro movie includes a badass military-march version of The Red Weed.
The instrumental version of Spirit of Man is surprisingly moving.
Fridge Brilliance - Where is the rest of the British armed forces when their homeland is under threat? Probably fighting in other parts of the world against foreign powers taking advantage of the Martian invasion!
It might seem baffling at first glance that for all their advanced tech, the Martians seemed focused on taking over Britain rather than going for a wholesale global invasion. But considering the clout and prestige The British Empire had on the world stage, defeating "the bastion of human civilization" would be a demoralizing blow to the rest of humanity. At the same time, however, the Martians as simply too desperate and few to mount a large-scale invasion that a victory in Britain is enough for the immediate term.
Most Wonderful Sound - When playing as the aliens, destroying an enemy base is marked by the Martians belting out their trademark "ULLAAA!"
"The Martian base has fallen sir!" and it's equivalents.