You don't have anything to lose by getting stuck in! There is very little difference between the A-level maths specifications of the regular JCQ boards (AQA, Edexcel, OCR) so you don't need to feel tied to any one exam board. The
content for A-level maths is specified by the Department for Education , so they are all much of a muchness. This might be different for other subjects, eg maybe for history, one exam board might offer topics that you prefer. However, for maths they're so similar that it really doesn't matter which board you choose.
Structure is 3 x 2-hour papers.
The first step would be finding an exam centre which takes external candidates. Start here:
http://he-exams.wikia.com/wiki/Finding_an_exam_centre . If there is a convenient and affordable exam centre then it might make sense to just go with the exam board they use.
Regarding when you book exams, this all depends on which exam centre you use. Actually from the exam board's point of view, it is possible to enter you even the week before the exam. However, this is much more expensive and inconvenient for the exam centre, so some will not take late entries. Most will want your summer entries to be made in January, but some centres have their own deadline which is in December. Again, depends how far you're prepared to travel and how much you're prepared to pay. The few dedicated independent exam centres which specialise in external candidates are generally very helpful and will take late entries, whereas schools which are doing you a favour by fitting you in alongside their own students, often won't.
Hard to say - it's so individual, and it depends how much maths you covered on your previous course.
If you do CIE International A-levels, there are two big advantages:
1. They have exams in June and November, so maybe you would be ready by November?
2. Retains the AS & A2 format, so you could take the AS this year and get it out of the way.
CIE is the sister company to OCR so I expect their A-levels will be similar, but you'd need to download both specifications and compare to check. More on
CIE A-levels. and the page for
CIE Maths A-level.
Alternatively, if you decide to stay with regular A-levels, you could do the AS this year anyway, as practice and as a way of checking your progress. This would give you a solid grade in the bank if you want to apply to unis next autumn.
CIE Maths AS is 2 x 1 hr 45 min papers
CIE full A-level is 4 x 1hr 45 min papers
Regular UK A-levels are 3 x 2 hr papers.
Another advantage of the CIE A-level maths is that you can choose which order you take the applied papers in, so it's more similar to the current A-level spec. If you like Mechanics but not STats, you can specialise.
I think there is no substitute for reading through the syllabuses though! They're not that scarey once you get stuck in. One really good thing about Edexcel is that they make loads of teacher resources and past papers freely available to all, whereas most of the other boards put this sort of material behind a teacher login. On the other hand, for maths you don't really need the teacher resources, you just need good textbooks and tons of practice! Past papers are freely available on file-sharing sites - just search TSR for help with that.
AQA has produced a helpful s
ummary of how A-level maths has changed overall which might help you decide between the current and new spec.
Do let us know how you get on!