According to the Manual Of ATS Pt.1, a clearance authorises an a/c to proceed under conditions specified by an ATC Unit. Clearances are based solely on known traffic conditions & are required for any flight, or portion of a flight, which is provided with an Air Traffic Control Service or Advisory Service.
Clearances do not constitute authority to violate any regulation established by the Dept. of Transport, CAA, or other appropriate authority for promoting safety of flight operation or for any other purpose. Controllers should not issue clearances which imply permission to breach regulations. This is especially relevant in respect of the low flying rules.
I learnt this "parrot fashion" (like many others) 43 years ago. Amazingly, it still sticks in my mind. Although it may have been reworded by now !