In my local Sainsburys, they have a bank of checkouts that are only for customers who've used self-scan. Then another bank of self-service checkouts, then finally a small number of manned checkouts. You can avoid queues by using the app, because those specific checkouts rarely have queues (and if they do, they move really quickly as people just have to scan their phone then pay).
It may well depend on the staff as to how well it is handled. It seems, from my observation, that my local Sainsbury's is involved in some sort of inclusivity project to employ people with mental disabilities on the tills, but some of them unfortunately have no idea how to deal with anything out of the norm and don't seem to have been trained when to call for a manager (in this case it should be for anything not following the standard process). I love the idea of doing that as work provides a feeling of worth to people even if it's basic work, but I don't think it's being done well, and it does affect customer service somewhat.
As one example, one of them kicked off an argument with me about whether I could swap a reusable bag I was buying because it had a bit of a thread pull which I thought might affect its longevity and wanted me to pay for it then take to customer services to swap it rather than just give it to them and grab another from the end of that till, which is plainly silly, and I was utterly gobsmacked they didn't want me to just swap it as that's what has happened in every other case in my whole life when I've found a damaged item at the till - indeed normally a "runner" is called over to swap it for me. It struck me that the manager should have been called immediately there was a dispute they were unable to handle amicably, and they weren't. So great project, not well run, and very much makes me want to actively choose the self-scan, unfortunately, as the whole thing was quite embarrassing and shows a failure of the system.
(I suspect the person concerned had some form of autism, and was interpreting "you can swap bags for life at the customer service desk if they're worn out" as a strict rule that you
had to do it that way once it had been scanned, which is a training error, really)