I have spent a lot of time looking at distance learning as a private candidate and most of them are not any good at all. They take 20x longer to reply to any help you need than a normal education institution, and they don't teach you anything, you have to teach everything yourself.
I'm taking A Level Maths and Further Maths as a private candidate too, I've decided to not buy any course for it because it's a waste of money, they just list topics you have to do and might send you the textbook needed, which you can do yourself. I did however get a maths tutor so I could get a reference for UCAS. However even then there is a lot of great youtube channels like Bicen Maths which are the same or higher quality than even my maths tutoring lessons (I only have one lesson every week or two).
For A Level Computer Science I ended up getting the fast track A Level course with Pembrokeshire (learn online) so I could get someone to mark my NEA since it's really hard as a private candidate. It cost me ~£990 which includes them marking my NEA, 2 past paper marking and marking some questions from each topic. Which is still over priced in my opinion but it's slightly reasonable I guess. I also went with one of the UK exam boards (OCR) and not international like cambridge because of how much more resources there is online which also justifies the £990 cost for me to be honest.
Some Universities don't care if you don't do practicals for science if you're a private candidate however I suggest you call them and ask.
Overall, do not spend money on these distance learning sites, you can do it all your self. Just buy the textbooks and go look at good online resources such as Bicen Maths, physicsandmathstutor and put time into your learning and you'll do fine. If you really do feel like you need some tutoring or help with past papers then use sites like MyTutor to get some help with a specific subject.
Use the JCQ site to find exam centres where you can do your exams and then contact them:
https://www.jcq.org.uk/private-candidates