I'll second the Acoustic and Bass Centre in London. Another good one is Coda Music in Stevenage and Luton - I've only been to the Stevenage branch, but the guys there were great. Both are more slanted towards "serious" instruments IMO though - kind of the £500+ range. And slanted towards "playing" instruments that sound good, rather than the £5k Gibsons which have stunning inlays and looks but sound hopeless.

If you're after more of a low/medium-end instrument (sub-£500) then a festival is probably a good choice. Hobgoblin and others often travel, so there's a bit of variety of shops. None of them tend to take the expensive instruments though (for obvious reasons) so you won't find the really good stuff here, just the good workhorses like Simon & Patricks.

Graham.