A Level in a gap year?

From an Oxbridge application perspective: there isn't much info out there about this specific situation, but two things Oxford do state are 'it is likely that offers will be made on the basis of three A-levels completed in year 13' (https://uni-of-oxford.custhelp.com/app/answers/detail/a_id/1385/related/1 although this is re taking an a level early, not doing an extra one). They also say that people who did retakes of exam based assessments have a lower chance of being offered a place (https://uni-of-oxford.custhelp.com/app/answers/detail/a_id/510/p/27/c/248). You're not doing a retake exactly but based on these two pieces of information it sounds like Oxford might look less favourably on applicants who couldn't meet the required grades in one exam sitting. It is only medicine that actually requires you to do them all in one go so assuming you're not applying for that (and given your subjects, you aren't) there's probably still a chance. Cambridge say similar stuff here (https://www.undergraduate.study.cam.ac.uk/apply/before/accepted-qualifications) in the A-levels section. So if you were interested in applying to Oxbridge and Physics did not turn out well then it might be a route to go down, though achieved A-levels that don't meet the requirements plus an extra predicted one that does would probably not be the strongest grade profile. It might then be even more important for you to do well on any admissions test/written work/interview to balance out the overall picture (and you have to consider whether you'd have time to prep all that and self study an A-level in a year. Especially as if you were to reapply to university and go for Oxbridge this admissions cycle, UCAS deadlines are fast approaching. If you wanted to apply with the achieved grade for the extra A-level you'd have to take two years out).

Not an option for getting into Oxbridge, but if you're interested in other Russell Group unis make sure you check out if any you're interested in are doing clearing this year for subjects you want to study, as that might be a way to get into a course and uni you like if you miss your grades now without having to take a year out to self study an A-level (though I can't tell if the gap year is your plan regardless or a contingency for if you don't get the grades). If you miss the grades it is also worth calling up your offer unis on the day to see if they might take you anyway, sometimes they do.

Also note that self studying an A-level can be quite expensive as you'd probably have to pay for exam fees and any tutoring you might need so keep that in mind as well and do your research about cost before you commit to the decision. Remember it is a lot of work as well especially in one year and by self studying you won't have as much support in the form of teachers and classmates as you did studying at college. I think if this situation does happen speaking to your college's uni person should absolutely be your first step as they can have an in depth discussion about next steps with you and will know more about your personal situation than I do!

I hope that helps a bit and good luck with results day!