Ooh I'm looking forward to the first time I get asked to scan my receipt in order to leave the shop with the goods that I have paid for at the agreed prices, hence concluding the transaction. They have no legal right to ask for that, or to detain you if you refuse.
There is an allowance for them to ask for a receipt if they have reasonable grounds to suspect you may be thieving, but asking it of everybody obviously negates that claim.
I'd be forcing my way out and if stopped calling the police. I fucking hate cunt behaviour by supermarkets like this.