Many people arrive in London alone to start a new job or a new life. People arriving into London are rather like sediment forming on the bottom of the ocean.
At first, there is no connection with anyone, and new arrivals tend to attract each other, rolling around, mixing, socialising, trying out this scene, that person.
However, with time they come to form relationships, develop history, and trust builds up, and commitment forms.
At this point, relationships start to become less dynamic, freeform and random, and more of a pattern builds up, and people stop seeking out others so much.
Think about it, if you have limited time, and you have a solid dependable synergistic relationship and friendships, why would you want to sacrifice them, for the possibility of trying out new relationships and friends.
If you go on to have a family, another layer of sedimentation occurs, until you tend to socialise regularly with the same group of families, with children, and a few single people, people that you have known for years.
You are taken out of the game, and new arrivals experience you as unavailable in the way that you previously experienced other established Londoners as unavailable.
So each generation of ‘searchers’ in London, tends to seek out and find each other, like freshers arriving at university, so that you find your friends tend to be people who arrived in London at the same point as you did.