Flyer Boy,
A-level Maths and Physics would be best but far more important is to do subjects you feel comfortable with and think you would be able to pass. It would be better to get a pass in Engineering than fail Physics. If you talk to your (current) subject teachers at school, they should give you a good idea of what is involved in the various A-level subjects and how they think you will find them.
There is some maths (not a lot) required for the commercial exams but nothing that isn't covered by maths A-level. In fact, most of it would be covered by the old maths O-level.
In my opinion, Physics is going to be more useful to you than Engineering but an ideal senario would be to do Maths, Physics and Engineering.
If you can cope with Maths and Physics A-level, you will cope ok with the commercial exams.
Good luck.