"there is nothing Ofcom can do to increase EMRP for Radio Caroline while (other) licences remain"

Thats not so.
OFCOM could easily permit up to 500kW on 648 in daylight hours and, after giving due notice, after dark too.
The ITU dont 'police' or control frequencies, it's simply a clearing house for discussion and negotiation. OFCOM (or other licensing authorities) can do as they see fit. Many administrations already do just that. The UK is one of the few whop act fairly and with consideration for other countries, and the various frequency plans agreed to 'by treaty' over the years.