Well, what do you want to do at uni? Personally, I don't think Art is worth it unless a) you really, really love it, b) you are really, really good at it or c) it is relevant to your future degree (art, architecture etc). Otherwise, it would definitely eat up your time - my friends who take it seem to be doing just art all the time! If you're just going to do a subject you can get an A in, do something else.
Out of the sciences Biology definitely has the biggest workload - the whole A-level is pretty much a memory game and mark-scheme-second-guessing. However, if you keep up with the work in class and make sure you learn the syllabus properly, and in chunks, it really isn't that bad.
Maths is pretty light on workload. You just need practice but other than that you can get away with little work. Chemistry's not that huge either - you have a lot of understanding to do, and a fair bit of rote-learning, but not as much as bio.
I *personally* would replace Art with something else - again, it depends on your future direction. Languages are always fantastic though; well respected and definitely very very useful in future life too. It depends on your aptitude at languages of course, but the workload's pretty light in my experience. German's awesome!
Further Maths' workload is entirely dependent on your mathematical ability. I'm finding it OK, but it does take up a lot of my time as I'm not naturally intuitive or mathematically inclined; however, if you have natural ability for maths Further Maths would be just as light a workload as maths.
Physics, I speculate, would probably be the same kind of difficulty and workload as Chemistry, though can't speak for it personally.
Geography also appears to be similar to bio in that there's a lot of things to rote-learn, especially case studies; but unlike bio there seems to be a lot of essays to do which I think adds to the workload. But everyone who does it says it is quite easy.