Eugh. I know we can't expect everyone to know all the coaster websites and stuff, but RCDB and Google really are the only 2 sites you need as a coaster enthusiast...
I live near Coventry and we're pretty much bang in the middle of the country, with good motorway and rail links. If you're driving, all of these parks are within 2 hours:
Pleasure Beach Blackpool
Thorpe Park
Alton Towers
Drayton Manor
Also within 2 hours are Chessington, Legoland and Camelot, but they're not worth bothering with if you're stuck for time. You could also spite Drayton if you only have 3 days for example, even though it's the closest.
So, pretty much all the actual major parks with unique stuff could be done, and from Cov you can do them all as seperate day trips without paying for accomodation.
However, you seem very dismissive and unwilling to travel further than 20 miles or so, so I don't know why I'm bothering posting this... Seriously, if you were to come all the way here, what's a couple of hours in the car, especially for major parks?!