While both are based on the same underlying principles; the Mechanics course is much more Mathematical and much more detailed than the Physics one. This is especially true as you progress up the modular ladder. Although a firm foundation in Mechanics does make most of the Physics syllabus much easier and much more comprehensible. The Physics syllabus is very weak on Mechanics itself, and at this level, one would find it much easier to convert from the Mechanics course to the Physics course and not the other way round.
Your Mathematics AS-Level has to consist of two Pure modules and one Applied; hence you will only be able to do one Mechanics module.
The whole Mathematics A-Level has to consist of six modules. Three Pure and three applied. Where the standard modules, and hence the ones you have to do are P1, P2, P3, M1, S1 and then it is a choice from M2, S2, or in some cases D1.
The Further Mathematics A-Level has to consist of another six modules. P4, P5 OR P6 are compulsory.
Here is a list of the respective modules that I am taking or have already taken:
AS-Level: P1, P2, P3, M1, M2, S1
A2-Level: FP1 (P4), FP2 (P5), M3, M4, M5, D1,
Newton.