The information on the note led to the invention of time travel.Think about it. At one point Millie refers to Ray, Toby and Pete as the "Creators" - creators of what? I see only one logical possibility - somehow, someone found that note and developed time travel with the imformation supplied. Given the huge risk of running into the Grandfather Paradox, this means that Millie either really, really cares about her job, or is really, really good at her job. The reason the ending is possible is because the information still exists because of Ripple Effect-Proof Memory - the Editors are still coming to kill them, just not right now.
The note was supposed to be blank.Think about it. When they see the picture of themselves sitting in the pub gathered around the note, one of them comments on the fact that the note is blank. Then the situation seems to be resolved when a spilled beer erases what was written on the note. The picture is depicting their moment in realising (or believing) that it was over, and they had conquered something big. This then spurred them on to accomplish their genius act, to create their great creation. Think about it a little more; people at the party are dressed as them from their first great adventure, so clearly that was important. The only other reason would be if they were in the timeline in which they were already killed in the pub, in which case their would be something referencing or alluding to their deaths. I'm sure at least one person would have been wearing a lot of fake blood.
The whole thing's a Batman Gambit by Cassie.In her time-travelling adventures, Cassie had met Ray and fallen madly in love with him. She learned that he would be in that pub on that day and set everything up so a relationship would be possible, including intentionally creating a minor and controllable time leak in the pub washrooms, so that she could determine when they would go. Ray, Toby and Pete's future fame was entirely invented by Cassie as part of the Gambit; the people at the party were actors Cassie hired. Also an actor was Millie; the plan was to have Cassie defeat Millie using time travel, but she couldn't get a signal and Millie became the mask, ultimately actually killing the pub. The reason spilling the pint reversed everything was because, without the note, that date was too hard to use and Cassie chose another date. Cassie realized this, and staged the whole "parallel universe" thing as an attempt to cover up her lie.
The idea on the back of the note was Ninja YodellerBecause Ruleof Funny.
The bad future is still the futureYep, depressing WMG. The future where London is decaying is the result of something unrelated to their antics.
Pete fell into Terra NovaParallel universes are canon, as illustrated by the ending of the film. Pete fell into Terra Nova, explaining why "our fossil record is totally wrong" and there were dead human bodies. Maybe he was in "The Badlands" which was mentioned but never seen on the show.