Since you are in London, Silvermans has got to be the obvious choice.
There's two good other otions. I live in Kent (not sure where in London you are, but if in south east London, or near the A2/M2 then these may be good for you) and there's Platoon Stores in Rochester (very good, and geared toward bushcrafters etc) and also Golding Surplus in Canterbury and Herne Bay (Herne Bay store is the bigger and main one. But I've only been to the Canterbury one since I work in Canterbury) which has frequent odd things. You can Google either for an address.
Don't know what you're after, but I would suggest Platoon Stores in Rochester in the first instance - closer to London and bigger than Golding (and Rochester has a splendid castle - tallest keep in England - built by the same man who built the Tower of London, and I once spent the night in there on a ghost hunting night. Didn't see anything, but it was bloody terrifying).