A few mountain bike centres across the country include:
Swinnerton Cycle Centre, Cannock Chase, Staffordshire
http://www.dalesbikecentre.co.uk/ Yorkshire Dales
Large national and online retailers include:
In general, most cycling experts recommend using a local, independent cycle retailer who will provide informed and unbiased advice on what bike/equipment you need.
It is essential to take specialist advice on your choice of bike and to have the bike adjusted to size and riding position.
http://www.bikeforall.net/retailersearch.php will search based on postcode and find independent cycle retailers in your locality
It is important to get details of all cycle repairers en route before you set out on a long-distance ride.
lists details of cycle repair shops, company profiles and customer reviews
http://www.cyclefix.net/ gives details of mobile bicycle mechanics
If you cannot find a repairer in the area you intend to visit from these lists, try an online search for cycle repairers adding the relevant location.