I `came out of retirement` to do an A level again, a few years ago and i am much older.
There is nothing to stop you doing what i did, download the syllabus, buy the textbooks and other resources and do them yourself.
This is much easier than it used to be in my student days as the internet provides so much helpful info for you (mark schemes, past papers, you tube videos).
You can then pay to take the exams at a exam centre (note for subjects requiring practicals or one to one communication with an examiner as in foreign languages this may be difficult).
I think your timetable is tight, i spent about 500 hours on my Ancient History A level (grade A) and of course you are restricted in only being able to take the exams in May/June or November not just when you feel ready.
Enjoyed the course immensely and am now doing Classical Civilisation.
In short, perfectly possible, go for it!