Career advice. Is there much chance of work as a...?

I can't speak for UK, as their system may be different, but, at least in the US, for the first two, you may be better of doing an apprenticeship then college at least initially. Gym instructor (not sure if you mean like a school-instructor or work at a gym, if the latter, a specialized program in fitness could be equally beneficial to college). No clue on the third. Talk to the program advisers at the school you like--they should be able to tell you what you would need to do.