I don't know anything about London. But I go to the GSA for my undergrad, and can give you any info you need. The GSA is great, and the scene here is smaller than London's, but it's wonderful. It's easy to make connections, and since it's a smaller scene, everyone seems to know everyone else. There's also loads fo cheap/free places to show work, which is always a good thing. The GSA has great tutors. I'm in the photo department, so if you need any information, feel free to send me a message!