Y'know OP, TSR isn't the only sociable website online with young people. Have you looked into Gumtree yet? If you go to the London page and click on the 'community' section you'll find loads of stuff going on in the area and many people looking to meet new friends. You can always find events and classes being listed on there along with people looking to make new friends.
Another option is Meetup (google it) which is AMAZING if you want to meet new people seeing as it's the whole point of the website. Basically, it's full of groups of people with particular hobbies - say yoga, or photography, food, languages etc - who set up events online with open invites. Anyone can turn up as long as they can get there. Sometimes there's a small fee if there's materials involved or if the leader is qualified in their hobby and teaching others but it's not usually more than a few pounds. I've personally gone to a couple meetups from this website and it is both legit and enjoyable. I highly recommend this site if you like to try new things or just meet like-minded people!
Now Craigslist, on the other hand, is a site you should probably stay well away from. Whole different group of people on there