The riding round Bristol is great, especially in the summer. Think woodsy singletrack rather than open moorland stuff. When it’s wet you’re better off sticking to the surfaced stuff in Ashton Court and Leigh Woods.
There’s more info about the local trails and a couple of route guides here:
http://bristoltrailsgroup.com/
As other people have said, bike theft is an issue around here. It’d be well worth picking up a hack bike for everyday travel – try Bristol Bike Project, Lifecycle UK or the Bike Jumble (first Saturday of the month at Mud Dock). And don’t leave your pride and joy unlocked in bike stores or communal areas.
Bike theft capital of the south.
Apart from London, the Home Counties and Cambridge.
http://road.cc/content/news/36808-police-stats-reveal-uks-recorded-bike-crime-hotspots-how-does-your-area-fare
Where people ride bikes, you get bike thieves, sadly.