Cambridge english lit grade

You got amazing results, and they care more about A-Levels than GCSEs (Although the GCSEs do still matter). I promise you that there are people with much lower GCSE lit grades doing lit at Cambridge.
I just finished the final stage of my cambridge application (sidney sussex), and I can tell you that yes, they do provide you an "Additional Interests Form" to disclose that kind of information (You just need a pdf of the brain tumour diagnosis- I sent in my visual impairment and autism assesment. It's for anything disability or neurodiversity related).
Sorry to hear about your brain tumour, hope you're alright. To sum up: Your grades are amazing, no need to worry on that front, and yes they will definitely take your brain tumour into account.

P.s. If you can work it in as a brief sentence in your personal statement it might make you seem more appealing as a candidate. Something like "When I was diagnosed with a brain tumour, the (emotion you felt) pushed me to (write a story or poem, enter a competition you won/did well in, or otherwise some achievement)". Try phrasing it like "I'm not asking for pity, I'm showing you that I've overcome something serious and it taught me something subject-relevant". You don't have to if you don't want to, just might help you stand out- every autistic in my school is applying to cambridge, but I don't know anyone with a brain tumour applying.