December 26 (or Boxing Day) is traditionally the start of the winter sales in the UK, when items are heavily discounted. In fact, bargain hunters can find some items reduced up to 50% off their pre-sale price.
Shops advertise unbeatable offers, with prices slashed (= cut) or sold at give-away prices. Many shops keep the sales on for a long period of time in order to clear stock ahead of new spring arrivals. You can often see signs saying Clearance Sale and Everything must go!
The best bargains are to be found at the beginning of the sales. Some people queue up overnight before the opening of the sales in order to snap up the best bargains. "Armchair shoppers" start shopping online even before the sales officially start. This year, there have been reports of record numbers of people through the doors, and huge amounts have been rung up on the tills in the first hours of trading. It seems as if the British are spending like there's no tomorrow!