ok so my friend had kind of similar issues but not to the same severity - she got all 9s at GCSE for the subjects she took for A level but scraped a 4 in maths and underperformed in other subjects, especially considering our school is high performing. She’s predicted 3 A*s and did her interview for Cambridge, and overall is doing super well now.
I know your situation is kind of worse but it is still savable. if you can’t retake all your GCSEs I suggest to retake them strategically, ie. retake subjects you’re still doing at A level (if applicable), Maths, English. It’s better to get good results on a few retaken GCSEs than retaking it all and getting a mediocre set. You could also consider discussing with your teachers retaking Year 12 so you have 3 years to do your A levels + GCSEs.
The majority of universities offer a “contextual offer” for “mitigating circumstances”, which is often a reduced offer by 1-2 grades. Even though your A Levels are good, and contextual offers are largely based on criteria you might not fit (look them up), this means that there’s a good chance unis will be less harsh on your GCSEs. Your school writes everybody a reference letter and some unis also have a “mitigating circumstances” forms or something. Basically if your school is good they will put everything you just said into the reference to persuade the unis as much as possible you deserve to be considered specially.
I will also say it looks really impressive for a student to go from those **** GCSEs to top A Level grades. You could even mention retaking GCSEs on top of learning a levels in your personal statement to show your study ethic and time management. Bad GCSEs and good A levels are much better than good GCSEs and bad A Levels
For now, participating in Access programmes like the Sutton Trust summer schools is great as well as entering essay competitions etc. Remember that although you might not officially meet mitigating circumstances often just taking the chance and emailing about your personal circumstances can get you in anyway. Overall retaking maths and english is a good idea because a lot of unis say you have to have a certain grade to go, but it’s not necessary to retake any others.