Flying into Cambridge

The closest international airport is (London) Stansted. A 30-minute drive from Cambridge, it is well connected by bus and train. There are cheap flights to many European airports through budget airlines (see Ryanair and Easyjet, but also Air-Berlin and many others).

The next one is (London) Luton, a 50 min. drive. It is well connected by bus, not so much by train (via London). A smaller airport, it also hosts some Ryanair and Easyjet flights.

From Heathrow and Gatwick there is also direct bus connection to Cambridge, but it is long (2h30 Heathrow, close to infinity to Gatwick). It can be better to get into London by train or underground, and then take the train to Cambridge at King's Cross station (see Thameslink for trains from Gatwick). 

(Stone Fly, Economic flights for Christmas. "Do they charge for the orange juice?" - "You bet.". By Forges, published by El Pais, Spain)