Have you ever used MIT's OCW? I've briefly taken a look at their 18.01 course and throughout C1-C4 of A-Level mathematics, you will cover roughly 3/4 of the material they cover there. 18.01 is a prerequisite for their two introductory physics courses, 8.01 and 8.02. Why not go through 18.01, seeing as it will probably help you for A-Level maths as well, and then try doing 8.01 and 8.02! Actually, you might see *part* of 8.01 in your mechanics modules as well!
I am not suggesting that you neglect your classes at school at all. Make those your priority. If you'd like good grades at A-Level, I suspect that a combination of your classes at school and past exam questions will be better preparation than either course. Nevertheless, I think you will be served well by OCW. All three courses mentioned are on OCW Scholar (basically, it's a special variant of the course that is designed for independent study) as well.