From the Cred Sheet, here's all my ranked UK coasters not including Merlin parks, including their overall ranking in my list:
Rank
Coaster
Park
46
Grand National
Blackpool Pleasure Beach
53
Cobra (Paultons)
Paultons Park
68
Storm Chaser
Paultons Park
93
Icon
Blackpool Pleasure Beach
95
Velocity
Flamingo Land
96
Kumali
Flamingo Land
97
Wilde Maus XXL
Winter Wonderland
106
Accelerator
Drayton Manor
113
Avalanche
Blackpool Pleasure Beach
115
Revolution (BPB)
Blackpool Pleasure Beach
117
Spinning Racer
Fantasy Island
118
G Force
Drayton Manor
119
Velociraptor
Paultons Park
126
Mumbo Jumbo
Flamingo Land
136
Flight of the Pterosaur
Paultons Park
147
Millenium
Fantasy Island
158
Rage
Adventure Island
162
Big Dipper
Blackpool Pleasure Beach
163
Ice Mountain
Winter Wonderland
166
Odyessy
Fantasy Island
167
Big One
Blackpool Pleasure Beach
169
Rhombus Rocket
Fantasy Island
176
Rockin Roller
Botton's Pleasure Beach
181
Infusion
Blackpool Pleasure Beach
182
Olympia Looping
Winter Wonderland
183
Dinosaur Valley
Wicksteed Park
186
€uro Coaster
Winter Wonderland
193
Shockwave
Drayton Manor
199
Hero
Flamingo Land
Also includes travelling coasters I've ridden in the UK, but oh well.
tl;dr - my answer is Grand National at Blackpool.