Having grown up in the UK and started my hunting and target shooting "career" there I cannot see any advantage the British could have gained over imports through the proof process. Their products were already priced out of the mass market or non existent, as in the case of clay specific guns.

A look at the Gunmark catalogs of the time, probably the largest importer, is revealing of the market dominace of imports. And it was not a matter of price either. At a time when a new Auto 5 cost about the same as a used Dickson Round Action more UK buyers preferred the Auto 5.