Something I hadn't also thought of before some of the last few posts, there are actually tonnes of attractions around.
The West is crammed full. Slimbrigde, Bowood, Westonbirt, Longleat, Monkey World, Centre Parks, Cheddar Gorge and Caves, Wookie Hole (even has it's own hotel), Grand Pier, Brean, Butlins Minehead, Aquarius, Cricket St Thomas.
Devon and Cornwall, as counties are almost resorts in their own right in places. Even there you have The Edon Project, Crealy, Woodlands, Camel Creak, Digger Land (I must go there at some point, sounds awesome!), The BIG Sheep.
I'm sure I've missed most of them as well. That's not including some of the other stuff like traditional museums (Tank museum, Haynes Motor, Concorde, Fleet Air Arm etc), the landmarks (Stonehenge, SS Great Britain, Berkley Castle, Cotswold Waterpark etc) all the Cathedrals (Wells, Bristol, Salisbury, Gloucester, Exeter), traditional seaside/fishing towns (Weymouth, Poole, Torquay, Clevedon, Weston, Burnham, Minehead, Lyme Regis, Exmouth, Salcombe, Budleigh Salterton, Ilfracombe, Teignmouth, Bude, Newquay, Polperro, Looe, Penzance, St Ives), and national parks.
So perhaps there really isn't any need? There certainly isn't a shortage of leisure activities to do. It's easy when you live somewhere like this to just look at the coasters elsewhere and wonder why we can't have such nice things, whilst neglecting what's here on your doorstep. I'm guilty of taking these things for granted myself and forgetting they're here.
If you live or are holidaying in Devon and Cornwall, there really isn't much need for a theme park. In The West, the southerners can get to Chessington, Lego and Thorpe in 1.5 - 3 hours up the A303 and M3. Us northern lot can get to Towers in 2-3 hours via M5 and M6, Chessington, Lego, Thorpe, and Oakwood in slightly less time via the M4. Bedford is going to be a bit of a ball ache though.