My advice would be to defer when you apply, rather than after you have the offer, especially as you seem pretty definite about what you want to do. I think unis respect the honesty of you telling them ' I want to have a gap year' rather than you mucking them about afterwards. I have a couple of friends who were all set to have a gap year, went to defer in march and the universities said no.
Apart from anything else, universities could have set quotas for gap year students, and have filled them up by the time you apply. Or by you telling them you wish to defer after they've given you the offer, they could have given your place to someone else particularly if you are applying to top universities that only have a limited number of places and hundreds of applicants.
It's up to you how you go about it, but if the worse comes to the worst and you apply for deffered enty and get no offers, you can always just reapply next year anyway, and with the lack of interviews (apart from medicine, oxbridge etc) it shouldn't be too much extra hassle.