Gil,
The reason you state is one possible reason for saying "made in England". The other reason is a gun was made in England for expectations that it is to be exported.
I have a best gun that is stamped "England" on the stock, the barrels and the water table. Also on the maker's label in fine print. This was in 1891 for a gun retailed by Read and Sons, Boston.
There is a proof mark that exists that is applied to guns of foreign manufacture being sent through the London and Birmingham proof houses. I would imagine the spurious "Made in England" marks would be put on Belgian trash guns in order to prevent the detrimental proof mark that denotes "foreign manufacture" from being added to the gun at proof.
In conclusion, old guns said "Made in England" because:
1. They were being made for export
2. They were forgeries being pawned off on the Brit public courtesy of the eurotrash in Belgium
3. Point 2 above, but instead of being targeted at Brits, it was to fool American hardware store patrons into buying a Liege gun because they thought it was a Birmingham gun.