Hey!
I was at the Biology open day the other day at Edinburgh (for those who were offered a place) and the guy told us that "you can take a gap year and we'll keep you a place for next year, so long as you let us know". I suppose you need to say you're doing something constuctive, but they were really cool about it. Then again as skegs says, it depends on your course (some are haveily oversubscribbed), however, there is no harm in asking. If you have an offer, speak/email the school's liaison officer and I am sure they will be able to answer you (thats their job) - at least the Bio one was really nice.
hope that helps,
magicgate