The problem for us, Dig, is that we don't know the pressures of your British loads--other than that they're within CIP specs. If they're well below the CIP maximum for a standard proof gun, that might mean pretty low pressure. If they're close to the CIP maximum, then they're not all that much different (about 800 psi) than our own SAAMI maximum of 11,500 psi.
I've never gone nearly as low as a lot of reloaders. I figure I'm safe in the 7-8,000 psi range, even figuring in some additional pressure for a longer hull used in a shorter chamber. And I've shot field loads of 7300 psi, 1 1/8 oz shot, 1200 fps--although not a lot of them--at pheasants. Seem to work fine (and produce very nice patterns) in vintage British doubles. But I've also never played around with anything older than 1920, except a couple sleeved guns from the 1880's.
