I studied up to M3 when I finished this year and it becomes very interesting from M2 onwards (M1 is just a starter like C1/S1/D1).
If you take Maths and Further Maths, the only compulsory modules are
-C1/C2/C3/C4 (Maths)
-FP1/FP2 (Further Maths)
and then you must have some additional modules for both A levels.
Here is the full list
Mechanics-M1/M2/M3/M4/M5
Statistics- S1/S2/S3/S4
Decision Maths- D1/D2
For regular maths A level
-Take two applied modules from the list (usually do C1/C2 and your first applied choice in AS year then C3/C4 and your other choice in A2 year)
For further maths A level
-Take 4 applied modules from the list (ones you aren't already doing in regular maths) OR 3 applied modules and then take FP3 (the only non-compulsory core module to my knowledge).
If you know that you enjoy mechanics already and would like to learn as much as you can then specialising in mechanics up to M5 would be extremely advantageous. Despite what people often say, that you should pick some from all the applied topics, it is much more rewarding to specialise in one topic and if you understand it then it is easier than picking stats/decision.
Statistics is also enjoyable (only after S1 though) but requires a different mindset to mechanics, where you are expected to learn formulae and then select the correct one for the problem before slotting in relevant data (this is what S1 mainly is though the final chapter on normal distributions is interesting).
Mechanics also requires you to learn formulae but far less. The vast majority of problems will require you to examine the situation of a system then solve the problem using
-F=MA (This comes up everywhere and I mean everywhere even for complex motions like SHM).
-Conservation of mechanical energy (Very useful for problems where F=MA would overcomplicate)
-Conservation of momentum
Mechanics becomes very calculus dependent from M2 onwards so as long as you are strong in core modules, you should be fine.
The real difficulty in mechanics is understanding the concepts.
Of course this all assumes that your school can teach up to M5 (which usually only private schools can because it's so specialised) so check with your maths department if it's possible.
If you can, then I would encourage you to specialise in mechanics where possible.