I second Music Room, simply cos Gina Le Faux does their fiddle department these days and she really knows her stuff.

http://www.themusicroom-online.co.uk/index.php/cPath/287_288?mrSid=eb0dc4ed335293c40dc3289479e80079

slightly further afield (Staffordshire) try Glen Titmus for good value and great service.

http://www.glentitmus.co.uk/gt_gtvhead.html

He also sells Bill Smith's fiddles which are among the nicest new made fiddles I've played in a while.