For large bore double rifle rounds, by the time you see conventional pressure signs you're in the Twilight Zone. Most folks accustomed to handloading for bolt rifles simply can't grasp this for some reason.

Most doubles are built in CIP member countries, and are therefore proved to CIP standards. In member countries, CIP standards are not industry standards like SAAMI's, they're law. CIP requires proof cartridges for rifles to produce a mean pressure equal to 125% of permissable Maximum Average Pressure. The large flanged nitros are low pressure rounds.

For example, the .470 Nitro Express has a CIP standardized MAP of 39,160 PSI. That means that, by law, the cartridges used to PROVE a new .470 Merkel, Heym, Krieghoff, Purdey, etc., produce 48,950 PSI. When do pressure signs start on modern drawn brass cartridge cases, and modern stainless boxer primer cups? Engineers at manufacturers like Hornady usually say around 62,000 PSI. By the time conventional pressure signs begin to appear on handloads, you're WAY past PROOF. Pressure signs simply can't be safely used as a guide when handloading for doubles. They can only tell you when you've already screwed the pooch, badly.

Personally, I wouldn't have anything proved in London until they can get the current idiot Proofmaster down the road. Better to use Birmingham.