If you do want to go to Oxford as I noticed was suggested earlier there are two brilliant, cheap and pretty quick coach services - The Oxford Tube and The Oxford Express. Prices were GBP11 return last year when I used them and it's an open return so valid for 3 months. You can get the bus from Grosvenor Gardens and they go from different stops but about every 10 minutes in the day. It takes 1-2 hours traffic permitting. It's quite a trek Cambridge to Oxford so may be easier to go from London.