It doesn't really matter what order you do the SAT/subject tests in. The SAT vs ACT thing is a matter of preference; try both practice tests on their respective websites and see how you feel about them. Neither requires subject specific knowledge except for maths, however it may be worth noting the ACT math section requires some topics only covered in A-level FM normally (some basic complex numbers and matrices topics), so that may be worth bearing in mind in terms of timing.
I would probably recommend that you aim to take the main SAT towards the end of year 12/over the summer before year 13, then that leaves you time to take the subject tests and/or resit the main SAT and/or take the ACT. If you've covered some basic material on complex numbers and matrices by the end of year 12 you could look at taking the ACT as well as the SAT around the same time.