To be honest, how could you know for certain that you want to be a pilot if you've never even had a trial lesson before? While the BA FPP is open to those with no flying experience, I'd be quite surprised if anyone made it onto the programme without at least half an hour in the air. It's not prohibitively expensive, and at least then you can say that you've done it and loved it as much as you thought you would.
I think there are several good reasons for doing the maths/physics tests. Firstly, skills deteriorate over time, and that's one of the reasons for having to apply to university within a certain time-frame (for some subjects) after having acquired the entry qualifications. Secondly, testing every candidate on the same tests establishes a level playing field; from which a much more reliable representation of everyone's current (and relative) maths and physics skills can be determined. What good reason exists for not doing that? If you're as good at each as your qualifications suggest you are then you won't have a problem. If not; my first point is reinforced. The last thing any FTO wants is to take on candidates who will need to relearn a lot of basic high school maths and physics.