If you do a search for
London Dealer Dealers you will find lots of threads, like this one which mentions most of the central London shops.
Aperture is worth a visit and Classic Camera, as well as Jacobs are all nearby.
Its also worth picking up a copy of Amateur Photographer which usually has plenty of s/h eqpt dealers ads in the back.
http://www.l-camera-forum.com/leica-...ic-camera.html