Here is my top 10 British coasters:
2 Grand National
4 Wild Mouse
5 Saw: The Ride
7 The Swarm
9 Big Dipper
^ I do actually really like Thirteen, it is highly rerideable and really good fun.
I'm surprised no one has mentioned X:\ No Way Out!
EDIT: If anyone is interested here is my bottom 5 (not including kiddy coasters, worst first):
1 X:\ No Way Out!!! (Infamously crap coaster, ride it every time I go for laughs)
2 Wall's Twister Ride (Reverchon Spinner...)
3 Infusion (It's an SLC...)
4 Air (Plain dull)
5 G Force (Would actually be a fairly decent coaster without the restraints trying to kill you!)