Not in London exactly, but Bryan Lucas ( Luke39UK ) is a member here and a specialist custom fountain pen maker. Worcester Pen Company
Then there is that other guy that customises Conway Stewart pens with silver carved barrels, but I can't remember his name.
I'm sure someone will remember though:biggrin:
(I don't mean Rich Littlestone in USA but he also does CS pens)
Henry Simpole just makes Gold and Silver overlays. I think he uses plain hard rubber barrels and caps supplied by Conway Stewart, and I know he works in conjunction with Bruno Corsini.