If you stay in England, add Blackpool Pleasure Beach and Thorpe Park to your list. Also check out this site (click on Roller Coasters above) and RCDB (which tends to have more pics posted) for more info on coasters in any area.
I-4: The scariest ride in all of Orlando!
Other parks I recommend:
Thorpe Park- Has Colossus, 10 inversion roller coaster and Nemesis: Inferno, a very sweet B&M Inverted. Also its really well themed and has an impressive collection of flat rides and water rides. Well worth a visit, its just outside of London.
Also Oakwood, its quite far out of the way- in South Wales, but I think its excellent. It is a small park set in a forest, but it has Megafobia, an awesome CCI woodie and Hydro, similar to Perilous Plunge at KBF. There's also Drayton Manor, near Birmingham, which has an ok Intamin Stand-Up coaster, Shockwave, but the real attractions are Apocalypse, an awesome stand-up/sit-down/stand-up floorless tower ride, Maelstrom (a smaller version of Delirium at PKI) and Stormforce, a great water ride.
I would say that the real essentials are Thorpe, Alton and BPB, but if you could get to the other two, they're well worth a visit.
Nemesis: Inferno 
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