For most of my ATC career, NATS/CAA insisted on a Class 1 medical certificate. It was almost the same as the CPL/ATPL medical, apart from the frequency of the ECG and possibly the chest X-Ray.
Passing the Class 1 exam, initially, and annually, was the best MOT you could have.
The Class 3 certificate was required for the PPL Course, which was part of the NATS ATCO Cadet Course.