Your Mum probably just wants you to get as many qualifications as you possibly can, in order for you to maximize your future options!
It's a tough world out there when the average house price is going to be many times your salary and I'm quite sure your Mum just wants absolutely the best for her child.
However, she sounds like she is not being particularly helpful in her pursuit of this matter, so perhaps you should bring up the subjct with her and discuss the options between you.
It would seem, by your A level choices that you are very clever, so to maximize this would be your best option. Joining the Police, although certainly not an easy option, is not going to use your 'scientific' brain to its full potential.
It is hard at 17 to envisage the rest of your life and your Mum is furthur along the 'life curve' than you, which gives her more of an insight into life itself.