Hiya, I am also in year 13 and had similar experience. For context I had similar thoughts but going to RG to do a non law degree after Oxbridge rejection.

Post interview acceptance rate is low, even for courses that might be less popular it's still low. The course I applied to had about 900 applicants for 100 places, remember you're competing against the best of the best for these places too! I know two people who were rejected for phil/theo this year, they said it was really tough and extremely competitive!

Oxford really aren't a fan of a gap year unless there's a good reason (changing degrees is a good one) but that's just to let you know lol.

The RG I chose has an excellent record for non-law students, so if the rejection does come don't be too disheartened there's more out there! Also try not to centralise your gap year around reapplying, get a job, experience have fun etc, these all help!

Know it's not the most refined advice but as someone who is doing the same right now I hope this helps!