If you're strictly looking for an offline college to attend an adult college, or a college that provides education to adults and 6th form students alike. An example would be the following in Essex:
https://www.uspcollege.ac.uk/The keyword 6th form shouldn't really be on the site unless it mentions it provides courses for adults as well.
After a quick search on Google, I can't see any popping up for Manchester, but they do exist.
Strictly speaking, you always have the option to buy the necessary textbooks for roughly £100 for each A Level and study yourself. I went with the online college because it provides tutor support and the course material is filtered through to the essentials, and I don't have to keep referring back to the spec to see what I actually need to learn. The added advantage of an online college is you could get assessed grades as opposed to studying yoruself, but you get the same with an offline college.
If you haven't done a Level 3 before, you shouldn't need to pay for the A Levels, but if needed you should be able to get learner loans. If not, they won't break the bank, but they can add up quickly.
The fees will depend on the exam board. I'm with AQA because I had bad experiences with EdExcel, and I had better ones with AQA. With AQA, you have to pay for the admin fee and the exam fee separately, but these fees all go through the exam centre. The fees with AQA add up to roughly £300 per A Level. If you decided to go with EdExcel, it's £125-ish I think.
People also seem to have good experiences with OCR, so they might be worth looking into.
Should you need to check, the fees are set by the exam board (not the exam centre), so you should be able to look them up on their websites. The approved exam centres for private candidates would also be listed on their websites.
The exam centres tend not to be that responsive for some reason. I have contacted 4-5 in my local area, and only 1 was nice enough to respond back on a regular basis.
I'm with Oxbridge, but that's after a week or so of questioning and going back and forth between online colleges. They charge roughly £700 per A Level. You can go for cheaper, and the cheapest I have seen is ICS Learn at roughly £340 for A Level Maths. The exam and admin fees don't go to them unless you want them to sort out your exam sittings. In my experience, it's a bit cheaper if you do it yourself.
So in total, I should be expected to fork out £1000 per A Level if I were to sit the exams once as well (it's different this summer because the exams were cancelled).
There are 5 online colleges that do both Maths and Further Maths (4 are unique, since one college trades under 2 brand names). 19 online colleges that do Maths (but only 16 are unique, since 3 operate under different brand names and can be for different audiences).
If you were to enrol, they have a deadline, and further maths tend to start later into the academic year for some reason. However, you can start maths almost immediately. You also have access to the material up to a certain period of time e.g. 24 months from when you enrol on the course.
As the tutors and study material for the A Levels can vary between courses as well as across colleges, the quality can vary as well i.e. the review of one course at one particular college might not do it justice with other tutors of other courses at the same college.
If you want the details of these online colleges, or my experience of mine, let me know.