Go for the subjects that you can do well in. Better having impressive grades in lets say english than a fail in physics/mathematics.
A B in physics and mathematics (higher paper) will be sufficient, don't feel that As/A2's are needed, they are not. (I think cabair want passes at higher tier mathematics and physics standard to be accepted onto AB Initio course).
Good luck.
P.s. onv. If you are good at maths and physics, take them - as for a degree, its your choice.