I was in London earlier this year, I conducted a search on "London Gunmakers" and ended up on a site that actually had a map with the makers marked on the streets. In four hours i visited 4 gun makers including H & H, all within walking distance of each other.
It is unbelievable and well worth it.
I particularly liked the .410 sidelock shotgun with the 36,000 pound price tag.
cheers