What are considered 'academic' subjects?

Traditionally academic subjects are generally held to be those that are primarily assessed by final unseen examination; essentially any subject which isn't primarily coursework based should be suitable (although as noted above, A-level General Studies and A-level Critical Thinking are rarely considered by universities when making offers). In terms of subject choices, I would note that virtually all degrees requiring A-level Physics will also require A-level Mathematics (and the ones that don't will require you learn the A-level Maths content in your first year modules anyway). If your goal is to go into e.g. physics, engineering, etc, you should plan to take A-level Maths. A-level FM would be very useful for these courses but isn't required typically.

For creative arts and design degrees, requirements are often less specific. They'll expect some kind of qualification at the 6th form level, which doesn't need to be A-levels (although it can be). Normally there will also be a requirement to submit a creative portfolio - this is usually developed while taking an art/design subject, or during an art foundation year (FAD) course. Sometimes they will specifically require an art or design subject at A-level standard or equivalent, although this is less common as a specific requirement (although as noted above, the portfolio requirement means many if not most will be doing at least one such subject).

Usually FAD courses are quite useful preparation for applicants to design subjects like fashion which often aren't as well supported in the A-level Art curriculum (which tends to focus more on fine art approaches than design approaches). It can also be a good "taster" to see if doing "full time" art/design work is for you, and getting familiar with working in a studio environment similar to how you would work on a degree in those areas. @PQ has made a very useful FAQ about FAD courses here: https://www.thestudentroom.co.uk/showthread.php?t=5017684 which I recommend you have a look through if you're considering potentially doing something art or design related at uni later.

I'm starting sixth form in September and still have time to choose my a levels.
As a future career, I'm interested in something involving Physics, but I'm also interested in Fashion Design.
A level Physics is my first choice, but I don't know what to pick for the other two. On the universities I've looked at, most of them state that they want three 'academic' subjects. What subjects are considered academic? Which ones should I avoid?

Maths, science, literature, history and languages generally speaking. Maybe geography too, traditionally at least