I was curious and decided to ask the CAA for my own knowledge.
The answer......
Medical Dept were uninterested. She said if the pilot was fit to fly but had a 'safety pilot endorsement' it meant just that. She said that it would be a licensing issue and duly transferred me to the licensing dept.
Licensing dept said.... "The 'safety pilot endorsement' means that the pilot cannot undertake the privileges of PIC solo AND MUST have another licensed pilot with them. IF the other pilot had the same 'safety pilot endorsement' THEY CAN fly together under Class two [2] ONLY". He recommended that insurers were notified.