Start with the end date. A code that expired yesterday will not be rescued by trying it in a private window. Seasonal events such as Black Friday and January sales turn codes over quickly.
Read the category. “Selected styles” and “selected tech” mean the retailer, not you, chooses what qualifies. Open the product page and look for a strike-through price or a note that promotional codes are excluded.
Watch the minimum spend. A £10 code with an £80 minimum is a poor deal if you only needed a £25 cable. Add the item you actually came for first, then see whether the code still helps.
Account-gated codes need you to be signed in. Beauty and fashion retailers often attach the discount to a membership. Apply the code after login, not before.
Finally, look at delivery. A generous percentage can be wiped out by a paid shipping option you did not need. Click-and-collect is frequently the cheaper combination.