Even though PMT (and other big chains) have sadly shut down, music shops still seem to be viable in the UK. We have....
The specialists who sell just one type of instrument: eg The Gallery, selling nothing but basses and bass gear.
Add to the list of specialists Just Flutes in Croydon, T A Craig (a violin shop in Sevenoaks), Laurence Dixon for Double bass, sax.co.uk
The quirky: Hobgoblin, for folk instruments. Richards Guitars for high-end acoustic guitars and mandolins, with each instrument set up before it leaves the store.
The traditional shops which provide excellent service: people here often commend single-store music shops, and which have built a good reputation. So, Rich Tone Music in Sheffield, and Rock Bottom in Croydon. As @SumOne describes them, such shops are "a go-to hub for musicians ".
When I go shopping for an instrument, I want a seller who knows more about the item than I do - not someone who got a discount because they bought 20 boxes of guitars, and needs to shift them quickly.