UK MATS Part1 CAP493 is fairly clear on this:
An Aerodrome Control unit provides services principally to aircraft flying with visual reference to the surface in, and in the vicinity of, the ATZ and operating on the manoeuvring area. It is normally a separate unit but may be combined, either temporarily or permanently, with an Approach Control unit.
An aerodrome controller shall not provide Approach Radar Control Services whilst engaged on Aerodrome Control duties.
Funny how many of the rules no longer apply when they are too expensive to be convenient.