If you're thinking of a gap year, go ahead and take it as it's probably the best thing you'll ever do. You learn so much about yourself and life in general, stuff nobody can teach you in a classroom, and your outlook and goals in life totally changes. It's also a good opportunity to start looking after yourself before you get chucked into halls at uni! Don't worry at all about forgetting stuff or getting out of the studying habit, they recap all the basics in 1st year (and there's always text books if you're not totally sure on something). Just be bold, phone them up and ask them if it would be possible for you to defer, the worst they can say is no! Once they've made you an offer they can't retract it, and anyway, you'd have no trouble getting a place next year once you've actually got your grades!
Good luck and get on with that travelling!