Mr Cad's new store is nice and easy to find, and the window display is something to behold. You may struggle with Leica and Voigtlander there though, I'd suggest checking his stock online beforehand. He's a lovely guy, but a tough negotiator. You'll get a great conversation if nothing else!
Aperture is always my first call. Got my Color Skopar 35 there around 12 months ago, and their gradings are always bang on. Fair prices too, with most bodies having been checked by their experienced techs and serviced if necessary. Again, great conversation. Unfortunately they have barely any Voigtlander stock at the moment.
Other used suppliers worth checking online beforehand are London Camera Exchange (you can set the used search on their site to The Strand store to be sure), and Camera World. Neither specialise in Leica but are worth a look as their customer service has always been very good in my experience.
Happy hunting!