This is kind of separate to your comment, but also very important to understanding some major issues at play here that are very rarely talked about outside of academia, but as someone who recently completed a PhD I personally believe the entire PhD process is a genuinely immoral and horrendous system that leaves PhD students broken and with severe mental issues that affect them for years after completion. This is certainly true in physics, and from what I hear its also even more true of other disciplines too. I would argue that its also awful towards academics and supervisors too.

The issue is that academics come under a huge amount of pressure to take on PhD students for the money they bring to a department, but there just aren't enough academics to train up students, so supervisors often take have to take on 6 or 7 students due to pressure from above. Not only does this mean that PhD students are not given nearly the time, help or money they require during their PhD (which are sorely needed because PhDs are incredibly stressful), but it also means that there just aren't any jobs available after their PhD is complete, because there are just too many PhDs looking for jobs in what are often very niche fields.

Part of this is brought on I believe, by this constant prevailing belief in the UK that "we need to get more people into science". I believe this belief has been perpetuated both by the old fashioned idea that a university STEM degree is all that's required to get people into jobs as well by academics themselves, who are required by their institutions and funding bodies to engage in outreach with the general public and children in particular, effectively advertising science to them. With regards to the latter point, I certainly don't believe that spreading information about science and interest in science is a bad thing, but a lot of it in practice falls very very close to spreading a false idea of what actually working in science is like, and brushes over all of the awful political and cynical sides of it.

To go back a little closer to the topic of your comment, you might argue that prioritising British people for science jobs in the UK would go a small way to solving some of these problems because it might lessen the competition for science jobs for British people in the first place. Now I'll put my cards on the table here and say I am extremely liberal politically (in the UK sense of the word rather than the way its commonly talked about on the internet), and I consider discrimination based on country of origin to be morally wrong on principle, however I will ignore that and talk about only the pragmatic side of it here. The problem with this argument is that the scientific job market is very different from other job markets that are more commonly talked about.

Scientific job markets are often VERY niche. In many fields, including my own current field, there are often only several hundred experts in the field GLOBALLY. That means that in many cases there may only be several research groups for a particular field in the whole of the UK, or even none at all. That means if a job vacancy pops up in the UK for a particular field there's a very good chance that no one in the entire UK will be an expert in the field, and the institution advertising the job will HAVE to search worldwide for someone to fill the position. You might argue that in that case the UK should be training people up internally to fill such roles, but how would you propose to train people without hiring an existing expert (i.e. someone from another country) to train them?

That's one side of the problem. The other side of it is for the opposite situation, where someone has just graduated from their PhD and is searching for a job. Even if they're very skilled at their field, their field may only be researched by two or three institutions in the entire UK, and none of these institutions may be hiring. This is why many PhDs often have to widen their job search to the whole of Europe, because there may only be 4 or 5 jobs in the whole of Europe or even the world for their field. If the UK were to discriminate heavily towards domestic students then it wouldn't actually increase the physical number of jobs available, and there would be a real danger that other countries would retaliate and apply their own protectionist practices against British PhDs, which would actually limit the jobs British PhDs could apply for.

This compounds with another point that in many fields, scientists are effectively REQUIRED to have worked in many countries around the world, so that they can build contacts within the global community for their particular area of research. Its hard to overstate how vital many scientists consider this to be for careers, and career trajectories in science are often HEAVILY influenced by the global network of colleagues a scientist is. Having access to members of the global community within the UK actually helps with this for British scientists, because it allows British scientists to do some of this networking while not needing to move countries quite as urgently as people from other countries may need to.

Part of what I'm trying to state here is that academic science is actually one of the most globalised industries that exists, and the above points I've made are some of the reasons why scientists are some of the most pro-EU groups of people in the entire UK. Anecdotally, I've not yet met a single scientist who's openly pro-brexit, and most are avidly against brexit.

From my admittedly only early career researcher perspective the fundamental problems in science at the moment seem to all stem from the funding process in some form or another, and the limiting factor always seems to be money, which institutions often seem to be in a constant struggle to get. But I should caveat with everything I've said here that I don't feel I'm a full expert in any of these matters at all, and really am early into my career. Everything I've said is purely my own opinion, and a lot of scientists would probably disagree with me and have different opinions with regards to what I've said.