Firearms sold to the Brit Military (including any imported like the S&W's and Colts) received a Military proof and mark,,the familiar crossed pennants marking in one form or another.
If the arm was eventually sold as surplus and before it could go on the Brit commercial market, it needed again to be proofed. This time marked w/commercial proofs
If imported directly for their commercial market, it needs to be proofed & marked first.
Certain gunsmithing operations require a gun to undergo re-proofing also before return to the customer or resale on the commercial market.
Proofing can be done at either the London or the Birmingham Proof Houses. They both follow the same Gov't rules, but have their own distinct markings in most cases.
Rules have changed as have the markings at specific times in England and it can be a useful way of dating,,at least generally, a firearm. There are always exceptions seen to those rules especially during war time.