Switching between courses totally depends on the uni. Within faculties it's normally fine, for instance I know plenty of people doing BA law (french/german) who switched to LLB etc fine. They just have to take tort in 2nd year (which, IMO, is a gift as a second year module!) along with the other compulsories as they dont take tort in first year.
As for switching to totally different courses, the general rule at notts seems to be that you have to take a year out and re-apply. Not sure if this is the norm though.