Since I have done A Level Maths as a private candidate, I can share the research I have done with you. I can't speak for the other subjects as a private candidate though.
Since the last time I have checked, there are 18 online colleges that offer Maths as an A Level (some colleges are affiliated with others, so you will get the same product at the same price, but at different institutions). Irrespective of the college, you will very likely need to arrange and pay for the exam fees yourself (unless they specifically state that they will arrange it for you, usually at 2x the cost of the course).
All colleges teach by exam board, so this is the first thing you would look into. Whilst there are 4 exam boards that do A Level Maths, AQA and EdExcel are the ones that are taught at online colleges, which is convenient because they are the most liked exam boards for Maths (I personally have an isse with EdExcel for maths, but people have a different opinion). The cost for the A Level course can range from £340 to £795.
I would not pick the course based on the college with the best review. I would pick the course based on the reviews for the specific course, as you can have a brilliant course and a terrible one both at the same college. This has more to do with the tutors than the college, so nothing is consistent.
Most online colleges will have roughly 20 assignments for you to complete; these are for your tutors to assess your progress more than for predicted grades. Your final grade for the A Level would be through the exam centre only.
The length of access for the A Level is typically 24 months from the time you enrol on the course (most colleges allow you to enrol any time during the year, but the exams would still be held in May/June). You usually go through the course at your own pace, unless there are live sessions involved (there are a few online colleges that do this for students, but the courses can cost £2000-2500 each). The recommended hours to study the subject ranges from 300-700, depending on the college. Personally, I would spend 300 hours learning the material and at least another 300 revising for the exam. 300 hours is typically 10 hours over 30 days, but that's a bit intense for one subject.
Calculators recommended for Maths include the Casio FX 991-EX (
https://www.casio.co.uk/catalog/product/view/id/1047/s/fx-991ex/category/154/) and CG50 (
https://education.casio.co.uk/products/cg50). The FX is roughly £30 new, and the CG is roughly £100 new. I bought both because the CG saves a ton of time drawing and checking on graphs (as well as the fact that I am doing Further Maths), but the FX is adequate for most parts of the A Level paper (I prefer this calculator because it's far easier to use).
Should you need to apply for your exams yourself, I recommend doing it early (e.g. before February) to avoid unncessary late fees. The fees for the exam are set by the exam body, but you will arrange the exams through the exam officier at the selected exam centre (somewhere local to you I guess). Even if the world go into another pandemic, and we require to go by predicted grades, you will still need to sign up your candidacy with the exam body in order to officiate your predicted grades.
After going through all the material, I would suggest doing as many practice questions and past exam papers as you can. The revision guides and exam practice books will be useful (unfortunately the only producer of such books are by CGP, which I am not a complete fan of). I would also look at YouTube videos and go on A Level tutorial sites in addition to speaking to your tutor if you don't understand a topic. When going through the papers, work on your weakest areas, as that's where you can gain the most marks and improve the most. If your course provides you with a question bank of questions to go through, so much the better (if not, signing up for one can cost £20-30 a month).
The alternative to learning the material through online colleges and offline adult colleges (if they offer them) would be to pick up the textbook for the exam body, and then apply for the exams yourself. This is significantly cheaper, but you won't get the tutor support.
For biology, there can be a pratical element involved and assessment on that can cost £1000 on top of the exam fees (don't ask; it's the same for all science subjects). It might not be neccessary under certain exam boards e.g. I don't think AQA mandates this in order for you to get the final grade (but may be necessary to get the full A Level); EdExcel would require you to talk about the theoretical implications in the exam paper, but Camrbidge/OCR will require you to have conducted the practical element and have it assessed. On the other hand, I would check the entry requirements of the degree course that you want to do and see if they require the practical element to be included in your declaration of your A Level results.
If the practical element is necessary for the degree course, I would try to do the A Level via your current college to save on fees and time.
You would only need 3 subjects to get into university. The subjects seem a bit random (except most of them are facilitating subjects) and there's not much consistency or intent. Are these solely based off your interests? If not and you intend to do a certain degree subject, can you specify what it is?
Also, what subjects does your college offer? The subjects you have listed seem pretty standard for a lot of colleges.