A-levels online will be quite expensive.
What is it you want to do?
Seeing as you have a BTEC (if it is level 3)
You should be eligible to do most degrees that aren't relevant to your BTEC with a foundation year.
E.g. if you did a BTEC in computing you could do say a French degree with a foundation year.
Also what are you GCSE grades like.
My opinion on self-teaching A-levels is don't do it. Most people who attempt it crash and burn. Get people on here all the time saying how they have planned to self-study 8 hours a day for like 9 months and that I am an idiot for saying it is not practical and you will struggle with motivation etc. etc.
You can self-teach some things easily. But generally speaking with A-levels is that they are stepping stones and the content is not very interesting. For example, if you are want to do a CompSci degree you might want to do A-level maths. A-level maths is a boring subject. It is extremely tedious to self-study it. Especially if you have very little love for mathematics other than getting the grades so you can got to university. In this example, it is so easy to lose motivation.
So, unless you are doing a degree that requires A-levels (can't think of any). I would not bother. I would recommened either an Access Course or Degree with Foundation year.
There is an argument that it is harder to go to top top universities with Access course. This may be true. But it is also true that universities like Oxford and Cambridge look down on people who retake A-levels or do them later in life so something to bear in mind. That being said, 99% of universities will happily accept Access students.