Foreign exchange student programs?

I'm going to be a freshman in high school this fall and I decided to look into foreign exchange student programs. There's a lot of fakers out there on the web claiming to be the real deal, but they're not. I would appreciate it if someone could give me a couple of good places to start looking to exchange to London or somewhere in Britain. This would be a very amazing experience for me and hopefully this question could help others out there looking for the same thing as me. Thanks a bunch!