On the cashier jobs - the thing is that they can rely on fewer and more trained/experienced staff. This may not be a bad outcome for those in the jobs, but it means less availability of casual work.

Which is not the same thing as no availability, but there used to be plenty of this kind of work to go around, such that it was the norm that 17/18 year olds could get it without particularly trying. That is not the case now.

Trying to bring skilled labour into the UK, or trying to get residence as an apparently useful and self-sufficient person, is actually quite difficult, so I'm not sure how all the low-skilled labour ends up here, other than large agencies in areas (social care, farm work) with known skill shortages.. being large agencies, the bureaucratic overhead isn't too bad for them, whereas for an individual or small company, it's more trouble than it's worth. There's no way you can just bring a cleaner over from abroad because you want someone to clean your house/office.

As to shouldn't be happening. Yes. But as for so many other things in the UK and its gradual slide towards a low-trust society. So it becomes a case of figuring out which battles to pick, which ones are genuinely impacting peoples' day to day. And I think collectively (not particularly a left vs right thing), the country has forgotten how to prioritise that.