Surf (Unilever) is one of the cheaper detergent brands in the UK, and big 130 wash (6.5kg) boxes of the powder are quite regularly on offer in discount stores like Farmfoods (currently 3 boxes for £36), and washes well although perhaps not as good as premium brands like persil at removing stains, so it is one I regularly use.
Only trouble is the fragrance is rather overpowering, although it is toned down a little in the dryer. My sense of smell has been slowly coming back since I had either really bad Flu or covid shortly after the first cases started appearing here, so I've just started noticing the smell again.
Daz used to be another more affordable brand that a few years back would regularly be on offer at <10p a wash; unfortunately, that also marketed on its smell, although was a bit less pungent than surf.