In extremis, if you are being moved on, there is always Waitrose!
There is a motorhome website somewhere devoted to overnight parking in various areas of the UK including attitudes of various councils and supermarkets. Sad I know, but it is updated quite regularly - or was when I happened upon it some time ago.
The Sidmouth Waitrose car park is being redeveloped at the moment and access for larger vehicles is a bit dubious, and spaces at a premium at peak times. The store is being extended.
Legally, it is private land and council officials have no say in you being there. You can be clamped or get a parking ticket - but only if the appropriate notices are in place warning you, which at Sidmouth Waitrose they are not. Just don't try it in Seaton Co-op!! £90 fine if you overstay by ten minutes all computer controlled and with numberplate recognition cameras. Reduced to £45 if you pay within 14 days. Yet another reason to avoid Seaton.
I've just loaded a bus map to help on another thread - Waitrose is shown -
Many supermarkets seem not to mind so long as you shop there and CLEAR UP AFTER YOURSELVES. So much anti-campervan feeling is down to people leaving a mess (some of it not for family viewing).