Why English??? Are the proof marks the one and only answer or could the barrels be 'proofed' in England and then sent back to France for assembly.
No. Illegal in the UK. In the UK in those days, tubes alone were provisionally proved before being used to build a gun, and were so marked (the mark that revdocdrew asked about above). Under British Proof law, only guns can be definitively proved, never just barrels, and the law requires both barrels AND action be marked after definitive proof. I tried to make this point in a previous thread about a Russian double that had marks on the flats that purported definitive London Proof, but was not marked on the water table, meaning that the barrels weren't original to the action, or the marks were spurious. That isn't the case with this Guyot, which has the correct marks on both barrels and action (note the crown over V "view" marks on the water table).
This gun was made long before the "Not English Make" mark came into use, so proof in London doesn't offer any evidence as to the origin of the gun. A foreign gun proved in London in those days would bear the same marks as one made in England. I think it more likely that this gun was made in France for an English customer, or was earmarked for sale in England by a retailer there. Clearly a quality gun.
.....If I was looking to buy it I'd have to ask myself is that purty box worth 10 grand ?