People on London trains can really piss me off. There a few types of offenders...
Firstly, people who sit on the outside seat and then tut when you say "excuse me please" to sit down on the free seat by the window. When there are about a thousand people all battling for an inch of space I think it is rather rude to assume it is acceptable to take up two seats all to oneself.
Secondly, people who don't move down. Again, when there are a thousand people all trying to squeeze into one carriage I think it is extremely rude for someone to stand half down the aisle and tut when you try and squeeze past them to make more room for the folk stuck on the platform.
Thirdly, (and this is more of an "I laugh at you because you are fucking stupid" kind of annoyance) people who run down the platform to get on the train that isn't due to leave for nearly another ten minutes. And even more annoyingly the lemming effect of others noticing someone running, and starting to run too. Chill out people! These types of offenders are usually those who do the commute wearing scruffy trainers with their office attire, and possibly carry backpacks. Another of my pet hates. Urgh!