I spotted loads of bottles of barbecue lighting gel at my local Asda, reduced from £2.25 to £1.50. I picked up four, and noticed that when they were scanned, they came up as 2p each on the till!
Sadly the checkout assistant wouldn't let me have them at this price and charged me £1.50 each instead but maybe you'll have better luck!
I was tempted to pick up a load more bottles and put them through the self checkout, but this would have meant removing all the reduced labels in order to get to the barcode, which would have looked a bit dodgy! Also it's likely that the purchase would be flagged for approval as it's age-restricted, at which point the assistant may notice the odd price.