I had a handy little chart (god knows where from) that said what subjects my A-Levels would get me into at each college, and how over-subscribed those courses were. And then I picked Downing because it was undersubscribed for Politics and had two rugby teams. I really think the only mistakes you can make are Girton if you don't like cycling, and St Johns. No-one likes Johns. Some people know what they want to do. Downing and Johns win the rugby, Downing, Caius and Trinity win the rowing, Trinity churns out mathmos, Downing gets all the best law-firm dinners.
Seriously, If you're going to get into Cambridge, you'll make your own way. You'll probably get through clearing after all the stress anyway. Just go with it. Y'al be reet.