Just make a trip to Alton Towers, Thorpe Park and Blackpool and you've basically hit the majority here.
At Alton Towers; the more unique coasters are found here, such as AIR (Worlds first B&M Flyer), Oblivion (Worlds first vertical drop coaster) and even Nemesis which is widely regarded the best in the country! There is one significant flat ride which is Ripsaw, but the latter are rather poor.
Thorpe is the best if you're looking for thrills nearest to "US" standards. For coasters, you have Colossus (previous record holder for most inversions), Stealth (previous record holder for fastest in Europe), Saw: the Ride which I'm sure you've heard of by now, and Nemesis Inferno which is a great forceful inverted coaster. If you're into flat rides you'll get along with Thorpe rather well, as they offer the best in the country (in my opinion), with Rush, Slammer and Detonator alone! Thorpe is not too bad for water rides, Tidal Wave maybe a little disappointing but Loggers Leap is definately one of the best log flumes around.
Blackpool is like travelling back into the past, where rollercoasters were slightly too dangerous and rather nerve-racking to risk on. But if this is your package, then you won't find anywhere else in the UK. It also has (in my opinion), the best water ride in the country "Valhalla", and if you're into this kind of coaster then you should be quite attached to the woodies. Great fun day out.