I ride with GMBC which is supported by alpine so may be biased, but my honest opinion is that they (in the georges x branch) are pretty helpful and go the extra mile, especially with any warranty stuff. Also have some great bargains from time to time, my bro has just got a hi-fi 29er full suss, with carbon rear triangle and full XT for £1000 (was £2k) so musn't grumble!
The workshop is pretty helpful too and have sorted me and my mates out on a few occasions.
In Glasgow wheelcraft is good too for rare stuff and wheels of course, and Craig McMartin (freeflow bikes) for more DH type kit.