Despite the fact that I'm a Haligonian, I'm actually going to defend Londoners.
I've lived in London since late 2003, and find that Londoners always seem to get it in the neck for being rude - more so than they deserve.
First of all, there are very few such things as true "Londoners" these days - most people living in London have travelled in from other parts of the UK or from abroad, and these people tend to be as friendly as anywhere else. Even with Londoners, most are generally ok and do not go out of their way to be rude (at least not compared to some really stuck up cities I've visited like Paris, Algiers or even Edinburgh). I've also seen real acts of kindness here that rate against anything I've seen in the North. The number of times people have asked me about the North or gone on about the wealth of their families to me I can count on one hand in over 5 years. Crime and anti-social behaviour in London is overblown, and I actually feel safer walking around central and south-east London at night than I have ever done elsewhere.
However, some things are different and you have to take it or leave it if you live here. London is not a city where you get to know your neighbours - I suppose the idea is that if you have enough friends throughout the city anyway then what is the point?.. Also, people do barge each other on trains and public transport without apology. It seems that in such a populated city, caring for other individuals' personal space is not a priority. Shop staff can be distant without being rude as such. When meeting new people, one of the first things you get asked is what you do for a living, something unforgivable in my home county of West Yorkshire for example. Dating and meeting women in London can be really hard.
However, saying all that, you can sometimes start conversations with people in pubs and cafes etc. People do not talk to randoms on the tube or London buses, but despite urban myth that happens rarely in other UK cities either. Some of the most rude people I've met were in the 4 years I spent living in Manchester - the last time I was there in late 2008 I was about to enter a taxi at just before midnight, just to be told by some lads also waiting for a taxi that there were five of them and one of me, and if I got into the cab they would drag me out and beat the shit out of me.Since my friend was waiting for a taxi at the same stop, I felt no choice but to step aside. That is rudeness, and something I have never experienced in London.