Hey there, TheHistoryphile,
I've taken a look around some university sites and newspaper articles; you seem smart so I'm sure this is all territory that you've already covered but I suppose it might be worth seeing if we both got the same impression at first glance. I'll stress that I'm not an admissions administrator or have any kind of competence in this; everything I say will be based on that search and what I've heard from my own inquiries in the past regarding admissions to Russell Group universities.
It's really inspiring to see your level of dedication and commitment though!
From what I understand based on my quick searches, retakes are generally permissible asides from subjects which focus on medicine, for which retakes are usually not accepted (I suppose these are subjects, after all, which require far more precision than others). Judging by your TSR alias, that shouldn't be too much of a worry for you :P
There are some newspaper articles which seem to suggest that statistically, retake candidates are disadvantaged over candidates which have sat a test only once. I don't suppose this is too surprising, really, because retake candidates are probably usually considered to be those who have developed their academic commitment or ability later than others of their age (rather than, as is in your case, those with exceptional circumstances which justify the retakes). The question to focus on, in my view, is how much are you disadvantaged rather than whether you are disadvantaged, and even then these articles seem to focus on medical sciences. It might be worth waiting for your results before definitely considering what you'd like to do: if they're better than you expect and meet minimum grade requirements, you might only be disadvantaging yourself needlessly by resitting exams.
I do vaguely recall being told at the University of Oxford that for certain subjects, tutors might prefer candidates who have taken a gap year if it has developed their cultural and subject understanding (I think History was cited as one of these subjects) - so if you do decide to take a gap year, it might be worth not only retaking your A-levels but also developing your interests in whatever subject(s) you choose to apply for at university, and perhaps even consider taking up a job related to the faculty. If you reflect your experiences on your personal statement when you apply next year, it might well improve your application considerably.
I suppose this is another important point to remember: by my understanding, Russell Group universities (and especially Oxbridge) take A-levels as a formality and normally would make their assessment on you considering the amalgamation your personal statement, your references, your interview performance (if any), your A-level results and any supplementary tests which they've asked you to take for your subject (again, if any). Candidates which have focussed solely on A-levels and haven't really done much for the subject outside of their exams are probably less likely to demonstrate the passion and motivation for the subject which Russell Group universities seek in candidates - so going over and beyond the syllabus is usually beneficial. Essentially, this means that A-levels aren't the be-all-and-end-all, and there's no need to aim for perfection if you meet the minimum grade requirements for the subject!
If it's of any relevance, I remember that one PPE professor said during Oxford's Open Day that PPE didn't mind the year in which an A-level was taken - although do note that this was the response to a question about early A-levels rather than late A-levels.
Again, the above is all non-official and just my first impressions. I'd encourage you to call or email university admissions officers for the subjects and universities in which you're interested. Without a doubt, they'd be happy to help and can give you the actual, official, expert advice regarding your choices (rather than the half-baked advice from my experiences, impressions and two-minute searches listed above)! I've done it in the past and the hardest part really is finding the correct telephone number online rather than actually phoning them.
One thing that you might like to bear in mind, though, is that undoubtedly, if you apply a year later, you should be prepared to answer any questions regarding why you needed to retake your A-levels. It might be useful to prove that you do have the academic and mental ability to cope with the stress and anxiety generally placed on students by attending higher-level universities such as Russell Group universities (good GCSE results, potentially?) - or you could always consider alternative courses like apprenticeships which the government has been so keen to promote in recent years.
And hey, I know that when I did my AS French last year, I'd predicted myself a B and ended up getting a theoretical A*. You never know: you might have done far better than you expect!
All the very best, and good luck!