As above. If it were me, I’d be trying my best to ace the entrance tests.
For Oxford anyway, screening for interview is a combination of the statement, GCSEs (in context) and the entrance exam. If you ace the entrance test it should get you over the line for an interview. And after that point it’s really all about the interview. If you apply, you’ll always get the opportunity of the entrance exam. While it’s weighted along with GCSE grades if you smash the test out of the park, I suspect that might over-ride any slight dip in GCSE marks.
Cambridge I’m not so well up on.
It may be worth looking at an admissions report for your chosen subject at both Oxford and Cambridge. Check if there’s a difference in how Oxford and Cambridge weight it.
Oxford are very open on their process. I’m sure Cambridge are too.
Overall though, although there’s a stated process I suspect if they like the cut of your jib someone may nudge the needle to get you in for an interview and just see how you do…
Is it not at least worth using one of your 5 UCAS choices to have a crack.. ?
Best of luck. 👍
Also, I see you’ve strong predicted grades for A-Level. That feeds in. There’s not much point waiting to apply after A-Levels, you might as well go for it now. If it doesn’t come off AND you get very good A-Levels that will overcome the GCSEs - you could consider re-applying. Nothing to be lost going in now except it uses 1 of the 5 UCAS choices.