I am in almost the exact same position as you (just slightly different subjects). I self-teach further maths but might be in a better position in terms of resources compared to yourself or anyone else self-teaching. I have a lot of contact with the two further maths teachers at my school and make sure to ask them any questions that I'm having trouble with, and they give me chapter assessments and stuff for 'homework' that has more loose deadlines than the actual FM class.
My school has a login for integral, a maths website, which contains literally every exam board for maths and further maths, and has some pretty detailed notes as well as practice questions and topic assessments (although I do not have access to these assessments, the teachers have to upload them to the school network for me to see them).
In addition to this, I've bought the textbooks for the topics I need to know for the exam (core pure book plus applied books) and they have some decent explanations (I use the Integral explanations primarily, but I check the textbook for additional explanation if I don't understand), but more importantly they have question which are more challenging than those on Integral. Since we're the first to do this spec, there's some mistakes in some questions which is frustrating but it's just something we have to deal with (but the exam boards really need to get their **** together imo).
Physics and maths tutor online is something I plan to use in the Xmas holidays when I do a bit of revision for the January mocks, since even if you have to use the old maths spec questions, the specification hasn't really changed much, and if you're worried that doing an entire FP1 exam (or whatever) would mean you see questions that aren't included in our new spec maths, then they have simple question topic banks where you can just do questions on things like matrices only, binomial distribution only etc.
I've been entered to do mock exams in January on further maths which means I'll sit the same exams and get my tests marked with a grade back, and this summer I'll be sitting the real AS examinations (my school still does them). I haven't been told anything about whether I'll have to pay to enter the exams, but I would assume I wouldn't, since my school hasn't made anyone pay for other exam related stuff like re-marks and people sitting extra GCSEs.
If you plan to sit the AS exam at the end of year 12 this year, I would suggest doing a good amount of catchup work towards the end of this term and over christmas because there's a fair amount to cover, and while it does have some overlaps and you do have enough time to learn it, you don't want to be stressing out in february/march where you should be preparing for your other end of year exams.