Which is where that excellent LCY-MAD-GIB idea comes into play!
I don't know if it's just my love for aviation, but I don't always see the train as the better alternative, if people understood that 20-30 minute check in was available (I'm thinking of routes like NCL-LHR, but it must apply for GIB too) with things like free fast track available on some airlines, along with food and drink on the aircraft (Bmi regional is one I can think of - and BA for breakfast)... Flying would be a much more attractive alternative.
The only problem with GIB-MAD is probably immigration as it definitely wouldn't arrive as a domestic flight