You will have to experiment to find the most appropriate gearing system for your London commute.
I suggest you hunt around for a frame that is compatible with most systems.
Look for a road frame with horizontal rear dropouts. I would also go for one with threaded eyelets for reack and fenders (mudguards). get these even if you dont currently use them, one day you will need them.
My preference is for generous tyre clearances, esp for winter commuting. A frame that takes long-drop calliper brakes can accept narrow or medium 700c tyres (usually up to about 32mm). Cyclo-cross style cantilever brakes can often accept slightly wider but of the 2 systems, I find callipers work better.

Once you find a frame, fit it with any components you like to make a totally custom bike. You may want to dress-down to avoid it becoming a thief magnet. I painted my old steel frame a garish green.