There were some pluses and minuses to choosing various UK airports, but in the end I decided to use London airports.

Ryanair turned out to be the best choice in terms of schedules and fares and airports. I decided to use Stansted on the outbound, because I wanted to go to Cambridge from Stansted Airport, and that is very convenient. For the return, it didn't matter so much which airport, but Stansted turned out to have the most convenient time.

Both ways, I got Ryanair 0++ fares. With luggage and credit card and check-in fees, the roundtrip fare for MAD-STN turned out to be 90.64 Euros. EasyJet looked to be about 114 Euros (not sure if any other fees would have come along after that), but not so convenient times and airports for what I specifically wanted.

Maybe I should have considered longer, but I didn't think the airfares would get any cheaper.