A U.K. PhD is going to satisfy "threshold requirements" that require, for example, a PhD to be considered for a position, even for a U.S. employer.
The hiring process in physics for PhDs is more individualized than it is in applications for graduate programs, and more still than applications for undergraduate programs and entry level jobs that don't require a PhD. Connections through your dissertation advisor, the match between your exact research subspecialty and the needs of the prospective employer, the mix of the current employees from Europe v. not, and the reputation of your institution, would all matter.
As an analogy, my son who graduated with degree in computer science a couple of years ago was recently hired for a job because he'd done work as a student in research led by a couple of his professors that was in the exact sub-sub-field of AI that his future employer was looking for (and the professors he worked for were known to the people hiring him). This was at least as important as any other factor.
It sounds like your immediate post-graduation goal would be to find work as a post-doc at a major research collaboration.
One way to evaluate empirically what matters would be to look at the biographies of post-docs at some of the major physics research collaborations (there honestly aren't all that many, a dozen or so maybe, and almost all have webpages for each of their post-docs) and see what educational backgrounds they had.
Consider emailing or calling some post-docs who got their PhDs at institutions you are considering in the U.K. to ask them what they think about the issue. Post-docs are not bombarded with these kinds of calls and emails. It's a pretty lonely life for the most part and usually the press and other big shots want to talk with the leaders of the collaboration and not the post-docs. They will usually be unreasonably generous in providing you with feedback. And, they have no incentive to be anything but completely honest with you, since you have no power of them and won't really be in competition with them for the same entry level jobs by the time you finish a PhD.
U.S. PhDs in STEM at top schools are still probably the most highly regarded (with a very large percentage of students coming from abroad to study in these programs), but there are plenty of other reputable PhD programs whose graduates do get hired as post-docs.
In research, as opposed to a tenure track faculty position, the employer will be more concerned about your personal "hard" technical skills than about "softer" factors like the reputation of the school and program you attended. From this perspective, the question is, in part, do you think you personally could get your "hard" research skills to the same level in a U.K. PhD program as you could in a U.S. PhD program? If not, that's a genuine, value added reason to pursue a U.S. PhD program.
One benefit to applying to the U.S. PhD program from the U.K. is that the admissions committee, at least, won't have to worry about your English language proficiency in your case. This is a huge question mark and subject to considerable efforts to conceal weaknesses, with many foreign graduates. This will give you a leg up to start with. British undergraduate degrees generally also have a better reputation in the U.S. than British PhDs do. Also, in interviews, a British accent just sub-consciously comes across as smarter to an average American interviewer.
