The statutory plate in the gas locker should already show the upgraded, maximum potential MTPLM. If it doesn't, as apparently in your case, then the caravan manufacturer has committed an error which should be corrected. Either way, because type approval may be affected, only the manufacturer is authorised to issue an upgrade.
Full details of the requirements relating to statutory plates can be found in The Road Vehicles (Approval) Regulations 2020 Schedule 1 Part 4 Item 18 where they refer to 76/114/EEC.
MTPLM most certainly has a standing in law and it is referred to in the same UK regulation that I mentioned above. One can find it in Schedule 1 Part 4 Item 48.
I have read through the legislation mentioned above three times and cannot see any mention of MTPLM?
Item 18 states;
18 Plates (statutory)
The technical provisions of: Directive 76/114/EEC as last amended by Directive 78/507/EEC, and where the vehicle is the subject of a multi stage build a plate is required on completion of each stage as appropriate.