All I can say is that I had 3 of those when I was young (bought one new in 83 and the other two in 88) and I should NEVER have sold them. They were the best sound I ever got.
JMP was Rose-Morris era and Jim was trying to get out from under them ASAP, changing the amp to the JCM. To that end, the US market (and I would assume the UK market) discontinued the 2203/2204 JMP by 1981. Parts of the EUR market still had them until ‘83-‘84 (including the 2203 used to record Ride the Lightning).
To that end, while existent, the post-‘81 JMP is an extremely rare bird throughout most of the world where they had JCM800s (Identical amps with different headshells).