Best university for Physics/Astronomy/Theoretical physics

Have a look at Imperial College, Oxford, Cambridge, and Edinburgh. They're probably the preeminent physics courses, and they have various options to pursue the study of physics which are all slightly different. Oxford has the option of Physics & Philosophy, which is functionally their theoretical physics course as you don't do the labs the single subject physicists do, and take specialist courses in the philosophy of physics and science (which are quite relevant to theoretical physicists). Cambridge is studied through Natural Sciences, where you'll take two additional subjects in first year along with physics and maths (such as materials science or chemistry). You can then focus mainly on physics or continue to have a slightly broader approach.

There are plenty of very good Physics courses outside of those of course. Southampton (which allows the top students to spend their final year at CERN or the Harvard-Smithsonian Centre for Astrophysics purely doing research), Birmingham (which also has an option to "intercalate" a year in computer science, which is useful background for potential theoretical physicists), and Manchester (notably the university which spawned the current graphene craze, also nominally the institution of Brian Cox, although I doubt he spends much time teaching there) are all well worth considering. Southampton also has some observational astronomy options, which are somewhat unusual otherwise in the UK as far as I'm aware.

UCL, KCL and QMUL are also good options, depending on your interests, and benefit from intercollegiate options across the federated University of London constituents (so even if a course of specific interest isn't offered by your institution, if one of the others from UCL/KCL/RHUL/QMUL offer it, you can still take it as an option). Exeter has a pretty decent physics course, probably only just behind those named above, and has some particular research strength in graphene (including theory) and exo-planet research (also biophysics).

The first set of courses would normally require one or more A*s, while the second might require an A*, but might be more flexible in giving an offer if you aren't predicted an A* initially or on results day if you miss your offer as well. The third are a little variable, UCL probably less flexible and with higher requirements (although, possibly a weaker course than the first set of universities), while QMUL and Exeter probably more flexible and with lower requirements.