I think it would have to be opt in.
Plus it would have significant overheads in running the system.
Some people go in every day... Would you really want that many emails from Tesco?
You pop in and buy a tube of smarties at 3:30after school on Monday....email
Grab a pint if milk and bread Thursday morning.... Email
Buy a box of chocolates and wine that evening..email
Weekly shop...email
Partner buys lunch every day ....email...email.... email... email... email
Of which you might need the receipt once.... Maybe??
Where as B&Q you might go to a handful of times a year. And you might have bought a lawnmower, where you do need to keep the receipt for the warranty...