The one I’ve used which is handy is the 10% off in store in Halfords. The site gives you a barcode on your phone that they scan in the shop.
Yes I know Halfords 🙄 but they are open after I finish work, are only a mile or so from here, and sell car bulbs and other boring stuff that I need.
Better discount than cycling UK which is only 8%
If you’re a regular need of car fettling bits could pay for an online membership fairly fast.
The 3 or 4% at supermarkets seems a great idea but I’m not organized enough to use it. My bad I know.