A discount code is a short instruction the retailer has agreed to honour. When you type it into the promo box, the shop’s checkout checks three things: the code is still live, your basket qualifies, and you have not already used a conflicting offer.
Most failures are not a broken website. The code may exclude sale items, require a minimum spend, or apply only to one brand inside a department store. If the basket mixes excluded and included products, the discount can shrink or disappear.
Percentage codes and fixed-amount codes behave differently. Twenty percent off a £60 order is £12. Ten pounds off the same order is worth less, unless the code has a low minimum spend and you were going to buy anyway. Always compare the final total, including delivery, before you pay.
UK retailers also separate staff discounts, student offers, newsletter codes and affiliate codes. They rarely stack. If you have two codes, try each one on its own and keep the one that leaves the lower total.
On Discount Code Finder we list the offer, the end date and the important exclusions next to the code. Copy it, open the retailer’s basket, and paste it into the promotional field. If the retailer rejects it, the terms on the deal page are the first place to look.