My wife also has Hapgood muzzel loader, but we belive it was made in the US. Warren Hapgoods book, The Hapgood Family has several references to those making guns.
i have a muzzleloading double barrel shotgun that is engraved on top rib hapgood. boston. and engraved hapgood on both lock plates. gun is of obvious high quality, total of four gold bands at rear of barrels, platinum blow out plugs, much engraving on all metal trim. barrel has english proof marks. cannot find any reference to hapgood being a gunmaker in boston in the 1800's, so i think this gun was made in england for a hapgood