This website is basically a directory of mountain bike trails in the UK which can be searched for via a map. It also brings up a list of nearby accommodation. There are reviews, some information, a social network of riders, photos, maps, videos and some bike related additional resources such as classified ads. A good site for finding local trails and venturing further afield.
This site has all the information you need for the various trail centres around the UK. The Forestry Commission who control all the forests in the UK, used to be only concerned with timber scales but in the last couple of decades they've branched out into recreation including cycling to great affect. Boasting 2500km of trails and some fantastic facilties, the website gives information about all the destinations, with levels of difficulty, maps and other resources.
With a good database of trails made up of reports of riding and article extracts from the magazine there is a huge amount of information available. The website design is a bit old fashioned so the trails are laid out like blog posts, but that doesn't detract too much. Comments from the community of bikers also provides some more up to date information.
Mountain biking is very popular in the UK. The UK has a huge range of varied terrains. With the diversification of the Forestry Commission's activities and the recognition of mountain biking as a growing benefit for the economy new areas are opening and facilities are being improved. With more people getting into the sport, many areas have communities and clubs often with well maintained and up to date websites. There is a great tool for searching clubs on the British Cycling website.