We know that if supermarkets were simply to agree to charge the same prices, this would be an anti-competitive practice or collusion and against UK law (whether it's a civil or a criminal matter or an administrative one, like fines for not submitting your tax return on time is unclear to me).
But it appears that there is no bar to one supermarket agreeing to match its competitors' prices.
What Tesco do is give you a voucher saying "£X off your next shop. Today your comparable grocery shopping would have been cheaper elsewhere, so here's the difference back." Sainsbury's do much the same IIRC (it's a few weeks since I went to Saiusbury's).
I got one from Tesco for £2.88 today. By force of habit I checked the long receipt to make sure all my multi buy discounts were there and after I'd loaded it into the car, I cross checked my shopping list to make sure I'd bought everything.
I noticed I'd forgotten to buy Dentyl, so I popped back in. Dentyl normally costs £3 a bottle, but they had it on special offer for £1.65, so I bought four.
It strikes me that I actually did myself a favour here; had I remembered to buy Dentyl the first time round, the notional of £1.35 saving per bottle would have been deducted from the notional £2.88 overpayment and I would have got less or nothing on the money off voucher.
So the correct strategy must be, first go in an buy all the stuff at regular prices, maximising the value of your money off voucher, and then do a second round where you buy all the stuff which is on sale, or 3-for-2 or 2-for-1 or whatever.
If there are two of you, you would just put the normal price stuff in one trolley and the special offers in the other trolley and pay separately.
Yes, this adds to the cost and hassle of shopping, but assuming you waste five or ten minutes going round and queuing up again, that still justifies getting an extra couple of quid on your money off voucher, I think.
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