Any point doing BTEC or access course if I won't study what I want at uni?

Like say you want to study academics like English or maths which you can't get into without A Levels

You can potentially go into an English degree after an access course. A few years back there were a couple articles about a formerly homeless big issue salesperson who did just that to go into an English degree at Cambridge
Oxford Cont. Ed. run a certificate in English literature that functionally is designed as an access style course for English lit at degree level:https://www.conted.ox.ac.uk/about/undergraduate-certificate-in-english-literature
But regular access courses are certainly possible to, particularly if they include English/literature sections.
For maths it's a bit trickier due to the subject requirement necessitating specific topics have been covered before which often aren't covered in access courses. So you may need to look at e.g. doing an access course with A-level Maths alongside or similar.
That said deciding between an English degree and maths degree is a significant thing in the first place as they are vastly different courses, and I'd suggest starting there

I couldn't decide between English and maths which is why I chose English and maths A Levels.