the roads in the UK are so bad, I'm ashamed. You want a road with not too many cars, but enough cars so that it's a decent road surface. B roads are okay if you're a roadie...
its a guide to long distance cycling in the UK, mainly on NCN routes, but you may be able to 'join' a few together to find a route?
Also, if you are a member of the Cyclists Touring Club (www.ctc.org.uk) you may contact members of the various 'sections' to advise on routes in their area, or even ask them to guide you through!
As I live in Bristol, if you contact me with your anticipated times of passing through this area I may be able to help you on your way?