What course do you want to do at Cambridge?
Does your school do AS levels? If so, it might be worth keeping History until the end of the year, so you can at least get something out of it; of course, only do this if it isn't going to damage your grades in other subjects.
From what you've said, I think if you're going to drop one, it should be history; if you want to do art in the future, you'll probably want to keep it up in some form anyway, so dropping the A level might not reduce your workload that much.