These are current police regulations:

"Hair must be neat and tidy. It must not be dyed in conspicuously 'unnatural' colours. As a uniformed service, hairstyles should be conventional and styles which may make members of the public feel uncomfortable or intimidated are not acceptable. You must have your hair above collar length. Where you have long hair this must be secured above the collar.

All non-uniformed police officers, unless engaged in duties of a specialist nature and members of the CID will, at all times, dress to a standard befitting the office they hold and the role they undertake."

I suspect they weould have been even stricter in the 60s when, up until the later 60s at least, even pop groups like Eric Burdon and The Animals had hair considerably shorter than Bacchus.