The British already produce steel shot loads suitable for use in guns that have not passed steel shot proof. However, those loads are restricted in terms of velocity and pressure. Also in shot size (limited to no larger than English 3.) English 3 are slightly smaller than US 4, which Tom Roster's steel shot tests on pheasants determined were not a very good choice on birds beyond 35-40 yards. Which means that those Brit steel loads wouldn't work on high pheasants, which would require heavier loads of larger steel shot. Or better nontoxic options like tungsten matrix.