Resit

How do I know where I can resit for a levels instead of my previous school? As in like just giving the exams

Which exam boards are you doing your A Levels with?

edexcel and ocr a

That makes things convenient.
If you go to the private candidate sections of the following websites:
https://www.ocr.org.uk/students/private-candidates/
https://qualifications.pearson.com/en/support/support-topics/understanding-our-qualifications/where-can-i-take-edexcel-exams.html
... both would point you to the following: https://www.jcq.org.uk/private-candidates
You would need to put in your postcode, awarding body (OCR/EdExcel). I haven't see Qualification type being a major factor though.
General rule of thumb is that if you can find an exam centre that's approved by one exam board, it's generally approved by all of them. However, there are exceptions so do check/
Do also note, getting in contact with the exam officers at some of these centres are a pain, so you would need to find a few of them as opposed to pin all your hopes on one.
Admin fees do apply as well as exam fees, but the admin fees vary depending on the location of the exam centre, with those closer to the cities being more expensive (for obvious reasons).

thank you so much! i have another question. do all universities even ucl, imperial accept resit students who only does the teams and doesn't go and attend classes?

who only does the teams and doesn't go and attend classes?
you mean exams?
I am not sure whether UCL and Imperial accept resits. They normally mention this in the entry requirements of the specific course page of the degree. If they explicitly state that they don't accept resits, then no. If they don't, they they might but you might want to double check. If they state that they do, then yes, although I haven't come across a case that they do.
When the unis see the grades, they come from the exam boards and not the colleges. This is why your grades are accepted with the same credibility as any other grade that they receive. If there wasn't a standardised method of examining or a standardised syllabus to go by, the unis would be asking a lot of questions - this is partially why it's difficult to transfer from one uni to another after completing a foundation year; their syllabi are usually different.

thank u so much! i looked through pricate centers and i have sa question about ucas. will i have to provide for reference for ucas application? i wont have any teachers so i wasnt sure what to do for reference. is it compulsory?