re-taking alevels at 19

so i have a btec level 3 diploma from college in health and social care, unfortunately my plans changed since i did btecs last year and now i am planning to study law at uni of liverpool.

their requirements ask for 3 a-levels (so not one btec equivalent to 3 a-levels) and i am happy to take a year to do this even if it might be difficult.
however my question is how do i exactly do this? i have heard of people paying to take exams and self-study which i think is the best option for me but i have no idea where i need to go to do this. has anyone gone through this before?

Before I answer you actual question, I'm just going to quickly check something. You say that you have "a btec level 3 diploma" and then later refer to the fact that Liverpool want 3 A levels, which you qualify as "not one btec equivalent to 3 a-levels". A BTEC Level 3 Diploma is the equivalent of two A levels, whilst the BTEC Level 3 Extended Diploma is the equivalent of three A levels. Is it the Diploma (worth 2 A levels) or the Extended Diploma (worth 3 A levels) which you have?
The reason I ask is that for Law LLB (Hons), Liverpool say here that they accept a "BTEC Level 3 National Extended Diploma" in (amongst other subjects) "health and social care". They ask for D*D*D*, which is a big ask, but I thought it worth mentioning. (They also accept the BTEC Level 3 Diploma, at D*D*, if accompanied by an A level at grade A, by the way.)
Anyway, as to self-studying A levels and then paying an exam centre to take exams, the first move would seem to be to establish what subjects you want to learn, and to then find a local exam centre (school / college) at which you can sit the exams. There is a search tool here where you can find some near your postcode (just select GCE for Qualification Type, and whichever exam board you want to use). Each exam board has a web page which provides some details about doing things this way. For example:
https://www.aqa.org.uk/student-and-parent-support/private-candidates
https://qualifications.pearson.com/en/support/support-topics/registrations-and-entries/academic-registrations-and-entries/private-candidates.html
https://www.ocr.org.uk/students/private-candidates/
https://www.eduqas.co.uk/home/student-support/private-candidates/

hi, i have a NCFE CACHE LEVEL 3 EXTENDED DIPLOMA IN HEALTH AND SOCIAL CARE that is in grade B, it is equivalent to 3 A-levels yes, however uni of liverpool doesn’t accept that and they ask for a different type of BTEC qualification or 3 standard A-level subjects. (as far as i understood when i asked them)

You can speak to the college you did your BTEC at about this. They’ll give you some advise on how to go about it.

they said they don’t take anyone over 19 to sit them privately, said i can only do it through online providers but couldn’t tell me any providers.