Hector.
Just a small detour.
You are quite wrong about contracts.
You are perfectly entitled to make an offer for the goods in your trolley once you get to the check-out at Tesco. The price displayed is purely, in legal terms, "an invitation to treat", which is to say "make me an offer". Clearly, putting an item in your trolley does not, and cannot, be considered to be entering into a contract, which is only concluded when the payment is offered and accepted. (Lots of case law on this).
That's what the law says, but what your fellow shoppers in the queue, some of whom will no doubt have bald heads and tattoos, say is a different matter!