All the more reason to go at less popular times.
I had a phase of trying the self checkouts when they were new, a few decades ago or so. They were so clunky, linear, and demanded that you conform to their slow timing that I stopped (despite the free stuff from the dance of it trying to roll back the conveyor belt and asking you to re-scan an item, after the item had already left the belt)
These days I'll use a self checkout maybe once or twice a year, if I'm unfortunately at a store during a peak hour with really long lines, or if I have one or two items and it actually seems beneficial to do so. They have improved slightly, but still suffer from obtusely taking your effort for granted rather than working with you. Most of the time I'd rather wait a few predictable minutes in line than deal with the hassles of using self checkout, like the non-negligible chance I'll have to wait for a worker anyway.
Also I know the legal environment is different in the UK, but people choosing to stop and have their bags/receipts inspected on the way out store is a peeve of mine. Especially when they build up a crowd that blocks the exit. Just exercise the smallest bit of self actualization, say "no thanks" and keep walking. (exception for clubs like Costco where they could theoretically screw with your membership)