I went to University to study Aerospace Engineering, and although I didn't like the course, the time spent there was very useful. It gives you a chance to develop key skills, (teamwork, leadership etc), that airlines are looking for in their pilots. Even if you don't decide on an aviation based course, choose something that you will enjoy, it doesn't really matter what as long as you take the experience and learn from the things you do there. I was told by a well known FTO at 18 that I didn't have enough "life experience" so Uni has really helped with that. However, from my experience, don't go to university if the thought of spending 3 years sat in lectures doesn't appeal to you. There are other things to do out there that are a good alternative, if not better. And I too know a fair few airline pilots without degrees. I also know a lot that do!
In regards to A-levels, in my opinion Maths is a must. All the flying schools test your mathematics at assessment so being able to do more than just basic calculations will stand you in good stead. Also take physics if you want to get a good scientific grounding. I did both at A-level, as well as chemistry, and havn't looked back. I hope this helps you make your decision!