I went on Vetcam last april, it was a good experience. Your school/college should have been sent application forms, if not, pester your form tutor/head of year to get some. There is some info on the cam website, think its in the veterinary faculty under open days. It costs about £100, but your school might pay (mine didn't by the way!) and involves a one night stay over in a cambridge college (was queens last yr, not sure if it is every year). basically you arrive, settle in, have some talks about cambridge, the vet school, entry requirements etc in a lecture hall. Then you get to look round the zoology museum (loads of stuffed animals, skeletons and stuff) before going back to the colege. then you have a (pretty snazzy) 3 course, sit down meal with loads of vet students and fellows, which is really good cos you can get loads of info from them! then you get to sleep, and the next day you get a bus ride to the vet school on madingly road, get a tour by a vet student and a couple of 15min introductory lectures on some of cambridge's research topics, before a finger-buffet and home.
Its a good experience, you get to meet some peeps your competing with (lol) and have a good nosy around the vetty parts of cambridge! Id say go on it if you can!