Mike,
I have a Powell on Damascus, coincidentally also made 1871. Peter Powell had it nitro proved just before I bought it - and it passed no problems even though the walls are thin near the muzzle.
The keys (as I'm sure you know) are the strength of the action and the thickness around the chamber and first foot or so of the barrel. My guess is Purdey's action should be fine as their models do pass nitro and presumably have a 'family' general design/thickness as well as using high quality materials. Again I would expect the barrels to be fine IF they have not been fine bored too much over the years. If they did start life as a 13 (and some did) and are now 12/1, I'd be pretty nervous. On the other hand, if they started as a 12, or even a 13/1 and are now a 12, you should be in with a pretty good chance. I believe the proof house would give 'an opinion' based on as much experience as you will find anywhere in the world, but of course you will have committed by then!
I hope it works out well for you.
John
