IMO, the sign to which you refer is a 'traffic sign'. It is prescribed for use on roads ONLY. On its own it does not convey any legal meaning and is there for information only.
IMO, the authority does not have the right to demand a prescribed penalty in this car park unless, as a minimum:
The conditions of use of the car park are clearly displayed and these carry the warning that failure to comply with specified conditions of use carries with it a stated penalty charge; and
That the sign states that spaces are reserved for BB holders only and that these are marked with ********.
