Blackpool, drayton manor. Also, you can catch a 5 hr train from london to Disneyland Paris. You also have parks like efteling and walibi (belgium and holland) closer to get to than it would be over here.
Coasterbruh, Voted best member of Carowinds Connection - Dukeis#1 Weekly, January 22 2005 --------------------
Alton is the best but can be a bit tricky to get to from London using transit. I took a high speed train to Stoke then a local train to Uttoxeter and a bus to Alton Towers. I think there is a direct bus from Stoke station too.
Thorpe Park is definitely worth going to, though I was a bit disappointed just because of overly high expectations. I was expecting something more immersive on the level of Alton Towers but it was more of a better themed Six Flags. Some good rides though (Swarm, Nemesis Inferno, etc.).