I understand that if any item is in a "Sale" then it has to have been offered at the higher price for a minimum of 28? days previous. So the big stores then put an item for sale at an inflated price for 28 days in some stores, then as if by magic reduce it to the 'sale' price.
OR
a box of biscuits that would normally retail at £4.99 is sold at £10 at several stores, if anyone buys one then more fool them, then 28 days (or howeverlong later) the sale price is £4.99, and the punters flock to buy the bargain!
Remember it is very rare to find real cheap sale items, Tesco, and all the rest, simply make mugs out of us.
My general rule is that if I wasnt going to buy it anyway, then I dont.