I think Oxford treats physiological scis in a way that is pretty much the same as the medics!
I.e. When shortlisting for interivew, both GCSEs(% of A*s) and BMAT results are taken into consideration(with the latter weighing up 2/3), as well as prediction of AAA and all that sort of stuff.xD
The good thing about GCSEs is that they don't take into account how many you've done, nor any other grades apart from A*s. However, in your case, I don't think this will work in favour for you.
Nevertheless, don't lose hope as a good BMAT will easily make up for it (plus, the competition isn't as stiff as medicine so they might lower the boundaries!).
Someone from my school applied for physiology a few years ago (with "not so amazing GCSEs, i.e. not a high A* %), but presumably did really well in the BMAT. She got an interview and then a place (St Catz if that helps.xD) so hopefully that shows that it can be achieved! =D
Also, I see you're a hopeful medic, I'd high recommend you to apply for Oxford's medicine course - i.e. don't apply for 4 med schools and physiology at Oxford (it's pretty much a guaranteen that you will NOT get in as youe PS will be clearly for medicine).
So you have to weight it up, it's either all medicine (may be plus a back up, but NOT a course at oxbridge), or physiological sciences at Oxford (plus various other courses).
i.e. oxbridge or medicine?
(and keep in mind that medicine at oxford is VERY competitive, the GCSE/BMAT shortlist will be really tough - you'll have to perform at least in the top 20% in the BMAT probably to get shortlisted).
If I were you, I'd apply for medicine at Oxford( if medicine is what you really want to do at the end of the day). Hopefully that helps!!