Building relationships in London is fairly easy - it's a cosmopolitain, open minded city, with lots of people interested in meeting others. One thing to bear in mind, however, is that you do have to make a bit of an effort first - people in London keep to themselves, unless approached by someone they know, and walking up to someone you don't know and starting a conversation with someone in a public place such as the London Underground or in a supermarket, can often be met with a degree of suspicion and an abrupt nature. The thing to do is to meet people in a social situation - there are lots of events going on around London all the time - a quick look at Time Out or Londonist will give you club nights, markets, speed dating, history walks, wine tasting, etc - all of which are great chances to bond with people in a shared love of a certain thing. Anything that "breaks the ice", and allows people to be relaxed around each other will net you some friends, and is far better and healthier, than sitting in the pub, waiting for someone to come up and say hello.
