Everybody seems to think that British gun proofing is some sort of a mysterious process and I did too until I went over there a few times and found out that it's really not.
When you submit a gun for proof pretty much all they do to it is take several different measurements to see exactly how the gun is beforehand. Then the gun is chocked up and fired with a proof charge in it and measured again. If the measurements don't change and the gun is not damaged in any way by the proof firing then it's stamped and pronounce "In Proof". There's a little more to it than that but not much, that's the basic deal.
You can get it proofed several ways. For awhile the Birmingham House was using some new proof standards that had been put forth by the European Union. But they were blowing a few guns up with those standards apparently as they are set very high. They were particularly having trouble with old big bore shotguns, the 8 and 4 bore proof standards were all what's called "magnum proof" and were damaging a lot of guns.
Alan Myers does a lot of cartridge testing and proofing with them. From what I understood when we spoke about it last, he's gotten them to start doing what's called service proofing again if it's requested. Service pressure is just getting the gun proofed a bit over what the cartridges you intend to fire out of it would create. I believe you actually submit cartridges to be tested for pressure along with the gun itself when you're doing this kind of proof.
The hardest thing about sending a gun over for proofing is the shipping of course. The Proof Houses don't receive or ship guns from overseas. If you want a gun proofed, you've got to get it over there into the hands of an agent or individual who can then get it to the proof house for the process. Guns over 100 years old aren't nearly as big a deal, they have a law very similar to our pre-1898 law that considers them antique. The problem with the customs people is that most of them don't know what the laws really are so any gun is a gun to them and subject to suspicion.
When I shipped the Tolley 8 gauge to my friend in England we got in touch with the gentleman who is in charge of gun imports and got the go ahead from him. We were able to ship the gun from person to person without a gun dealer being involved. It had two things going for it, it was over 100 years old and was considered an obsolete caliber. That's another part of their gun shipping laws, the obsolete caliber thing. If a gun is chambered for a cartridge that's no longer commercially produced it's considered obsolete. This includes stuff like 8 and 4 gauge and even short chambered 10 gauge guns as well.
My friend received the gun and the box had never even been opened by customs. We assumed that the man in charge let them know the box was coming through and gave it the nod of approval since he'd spoken to us and knew exactly what it was. I don't know that for a fact but I do know that we didn't have one problem with it.
Customs over there is just like customs over here, most of the people working in it don't know all the laws and rules. I've never shipped a gun to a gunsmith that I didn't have to explain the law to the person at the Post Office beforehand. Same deal with them, nobody had any idea what the rules actually were. We got a huge run around until we got the number of the man in charge and from then on it was no problem. I had slight problems with the USPS as well, they didn't want to ship the gun because they said they couldn't send it out of the country. I had all the rules and regulations printed off the computer and in hand, I laid it all out for them and they shipped the gun.
I'd imagine once you got rolling with Teague and the customs people got used to seeing boxes come from you to them you wouldn't have nearly so much trouble. And same deal with the USPS, if you were using your local office they'd get the process figured out pretty quick.
That's what I can tell you based on my experience and limited knowledge. Hope it was helpful.