It certainly was, and is, still possible to walk into the Proof Hopuse in Birmigham, England literally "off the street" and ask that a weapon be proved "on demand". You will pay a 50% premium on the normal price and have a wait of about twenty minutes.

With an uncommon calibre usch as you have, a contact some weeks before would be useful. It would be quite possible to send the gun via a courier service to the Proof House, anyway.

The British gun laws, BTW, do NOT apply to arms that are being submitted to proof so,I don't know if you would require any export or import formalities. I would wrtie both to the Proof House AND to the British Embassy to ask questions about that aspect.

Good luck with your rifle.