Hi Helena.
I was there at the weekend as I was at the WIMBLEDON final.
Thorpe isn't far from London. If it takes 2 hours to get there, it'll only be because of traffic. Getting anywhere in London is an utter ball-ache unless you're using the underground. It's also more geared towards thrill rides so has better coasters. It has a really good selection of rides and is close to London so I'd recommend it.
Chessington is really a kiddie/family park as is Legoland (which isn't really near London) and neither have decent coasters.
Alton Towers is epic. (I'm biased, I know lol) It's nowhere near London though. It's so unique and is very innovative with a few world's first coasters which are world famous. I'd definitely recommend it but if you're travelling to the UK, it's really worth going for 2 days and staying over. It'd be a real drag travelling up from London to get a few hours there and then having to go back. Also, there's no way you can get round it all and get on all the rides in one day. April, May and September are quieter but this weekend, the kids break up from school for the summer and it'll be rammed.
One thing I would say is go on the London Eye. It's absolutely brilliant and the view up there is awesome.
Also, the Olympic Park now has the world's tallest and fastest tube slide. It opened a couple of weeks ago but I didn't get chance to give it a go at the weekend. It goes around the big red tower sculpture that has the restaurant at the top. I think it costs £15 to go up the tower and then slide down. It looks amazing.
Also, Madame Tussaudes has a Starwars section now. If you're thinking of going, I'd recommend getting your ticket online beforehand as it's cheaper and the queues are hideous if you don't have a ticket. Same goes for the London Eye. In fact, as they're both owned by Merlin, you can buy a combo ticket which saves you even more. Thorpe, Alton, Chessington and Legoland are also owned by them but I don't think you can include them on the combo ticket.
So, May the force be with you this weekend and have an awesome time. Let us know how you get on and get some photos. [happy][up]