I wouldn't go for 'well-rounded'. I think a straight-up approach is the best.
Become the absolute best you can be at your area/field, nobody wants a 'jack of all trades, but a master of none'. In other words, pick subjects that relate. It'll make your two years at A-Level easier, and enhance your prospects after you finish.
But because you're unsure, the best A-Levels (employability wise) are; Further Maths, Maths, Physics, Chemistry and Biology. Yeah, the sciences.