For dentistry/veterinary science/medicine, the deadline would be the 15th of October. Besides, if you're thinking of applying to the oxbridge, you need to have your UCAS form submitted by the 15th October as well(for all the courses). On the other hand, overseas interviews will only be given to those who submit the Cambridge Admissions Form by 22nd September(I'm not sure about Oxford as i didn't apply there). Other courses' deadline, as someone has said, is on the 15th of january.
ditto to what sillyboy's said. The website for UCAS is www.ucas.com
(what an obvious web address)