<p>Switching major within a faculty isn’t too difficult (though a pain to schedule). Switching from one faculty to another is more complicated. You can certainly do math (or maths since you’re from the UK) in the Arts faculty (it’s the closest major to physics that can be done in arts, math is also offered in the science faculty, the program requirements are slightly relaxed in arts), and register in a few courses in the science faculty (not entirely sure about the details). Presumably, if you take the right courses, you might be able to switch to physics in science, though good luck with the red tape.</p>