girasol
Addicted to handstands!
There's a barrier indeed, can't get out of the area without scanning receipt, unless you force the gates open or indeed go behind someone else. (Sainsbury's Balham). There's a security man by the entrance, so I guess that would be the next barrier on the way outI am curious as to what the setup is at your Sainsbury's girasol. I can only imagine a train station-style turnstyle. Otherwise, what's to stop you walking past whatever barrier/check in place?
If someone is really determined I doubt they'd stop them as hands would have to be laid on people and it could get ugly and expensive for Sainsbury's. I think it's all more psychological than enforceable. Most people will comply: social contract... Then a very few won't.