I have never ever stayed on a supermarket car park for the night. Sounds horrible!
Agreed for UK but in France, both LeClerc and Carrefour allow you to overnight in their car parks. We did it in Dieppe Carrefour, then went shopping when it opened in the morning. Not my normal choice, but when you get off the ferry at 2330 there's not a lot of choice and Dieppe Aire is always full!