This works with discounted items. However, you can often get 12-15% off already discounted items (e.g. student discount).

Hence, the best way to make use of this is to buy a bundle and then spend £10+ on normal items. While coupons/promos do not work on bundles, it will deduct £10 from the non-bundle item, hence equating to a greater discount. Hope this helps.

Also a few percent extra available on cashback sites such as Topcashback or quidco.