2) Would a gap year be worth it in order to resit UCAT/work/travel/shadow a doctor to make sure I really want to spend the next 10+ years training to become one
Resitting UCAT isn't a way of figuring out whether you want to spend the next decade training to be a doctor (or actually being one in the first place). Don't take a gap year to sit an exam. Take a gap year (or gap years, pleural) in order to learn, mature and gain insight - and you can do so by working, travelling and just getting life experience. If you use years out of study wisely, this is never a wasted effort.