>>> Chris, do all supermarkets allow you to see the price history?
The supermarkets themselves might well not but Trolley.co.uk tracks prices exceptionally well. Where a product is sold in several supermarkets though, it will only show what the lowest price was at any particular time, rather than indicating the individual prices for different supermarkets. For example, if you take a look at this wine, you can see that the cheapest current price for it is £6.50 in Sainsbury's:
https://www.trolley.co.uk/product/casillero-del-diablo-merlot/JQK438
The price history function though shows that it's been available for £6 in at least one supermarket at a time for much of the past year, except for a couple of brief periods when you couldn't buy it for under £7.50, as well as for the past month or so (when it's also been at least £7.50 everywhere).