If this pistol was made in England, or for that matter in Liege, it would have DEFINATELY have been proved. Even if made for some notable person it would STILL have had proof marks.
The USA (for example) however DID NOT prove its weapons, so although marked LONDON on the barrel (which would as you suggest probably give a false lead to the buyer), this could possibly have been made there. There were some very nicely decorated pieces eminating from the States.
