As far as the UK goes, Thorpe Park and Alton Tower should blow you away. Their coasters are quite unique, and many of them are built to work with their suroundings (ex. Air, Nemesis, etc.) Both have an amazing blend of thrilling coasters.
You should also make a trip over to Germany if you would like theme. They have Europa Park, an amazing park which is strangely similar to EPCOT as well as Movie Park Germany with its theming equivalent to Universal.
I strongly suggest making your way down to Paris for some Disney as they feature some of the best Disney Coasters. Space Mountain Mission: 2 is the only Space Mountain with inversions, Crush's Coaster is pretty neat, and Big Thunder Mountain Railroad is definitely one of the best of the clones.
And lastly, you absolutly must go down to Spain for Port Aventura. It features amazing coasters and, being a former Universal park, great theming. Some of the highlights include Dragon Khan, arguably the best floorless coaster on Earth, and Furious Baco, a coaster that is pretty neat for being so flat (though it is somewhat off the ground, has three inversions, and oh, did a mention a launch of 80 mph!)
I am very jealous of your trip but hope you have a great time. Since your trip is 2 weeks long, you will definitely be able to enjoy these parks while still relaxing and mabye visiting Big Ben or the Eiffel Tower! Hope this is helpful!
Alton Towers is the standard-setter as far as UK parks go, there's nothing better in Britain. Other UK parks worth a mention are Thorpe Park, Drayton Manor, Blackpool Pleasure Beach and Legoland Windsor.
There's also a small park in the North East of England called Flamingo Land (My local park) which is about to open a new S&S El Loco with the worlds steepest drop in July.