My grandad is a doctor and when i was considering doing medicine a few years back he told me medicine is something you need to be 100% committed to, otherwise it wont work out.
My mum is similar to yours, she didnt get the opportunity to get a good education and now she is really pushing me to do my best, nad in the process she kept pushing me towards medical subjects.Not really because she wants me to do it, but because it is what interests her. She is only doing it because she doesnt want me to be in the position she is in now. However, she has always said that she will support me whatever i choose.
I am assuming tha tou a re going to start your AS courses in september? Have you chosen your subjects yet? Why not pick english and science subjects? then both options are open to you when you come to choose your uni course. This will also keep your mum happy for a while!
I dont mean to sound patronising, but you are still quite young and its hard to make such big decisions at 16, i know that my ideas of what career i want have changed so many times over the past few years. try and keep as many doors open as possible and most importantly, make sure that you are doing what you want to do. Your mum obviously loves you and wants to support you, and im sure she will whatever you choose in the end.