It is my understanding that the whole Class 1 OML / Class 2 unrestricted arrangement with the UK CAA is based on a risk assessment.
A pilot flying with a Class 1 will be a commercial pilot trained to a higher standard. Therefore the majority of risk on any flight will come from a chance of pilot incapacitation rather that pilot skill / ability, the stipulations of a Class 1 medical attempting to mitigate against this. If an individual cannot satisfy them then an OML will be issued to attempt to allow the pilot to fly commercially whilst not increasing the risk.
A pilot flying on a Class 2 however will be trained to the standards of a private licence and so less of the risk during flight operations comes from the pilot's risk of incapacitation, but rather from the pilot's skill and or ability. Therefore it is deemed safe for a pilot with a Class 1 OML to operate with an unrestricted ( Class 2 ) for non-commercial operations.