Best 'real world' stores I've been to are Academy Of Sound in Stoke on Trent, and Phoenix Soundworks which is somewhere in Northamptonshire. They both have friendly, knowledgeable staff who made me feel comfortable and not pressured into a sale.
As far as online stores, I've used Absolute Music, GAK, Gear4music, and effectpowersupplies.com and they were all great. I had a problem with an amp from GAK a few months after buying it, so they collected it, got it repaired, and sent it back within a week! effectpowersupplies were incredibly helpful when I tried to buy an out of stock item. Haven't dealt with customer services on the others.
Digital Village (DV247.com) on the other hand, I would avoid like the plague. I made a large order with them a couple of years ago, and spent the next few months being messed around with items not being in stock, then being in stock, then not again. They wouldn't even split my order into things that were or weren't available and kept lying about when they were due to receive them. I cancelled the order in the end and got a refund, which in all fairness I didn't have to wait long for.