The companies that fly to Madrid, from the UK, can be split into the standard larger carriers like British Airways and the ‘no frills’ airlines, like Easy Jet. Let’s take a look at them:
British Airways and Iberia are the ones I usually fly with; the latter you need to book well in advance to get reasonable prices.
British Midland – I flew with them this summer but bought the ticket through Opodo, which worked out cheaper. Opodo is actually owned by the big European airlines.

I often find that flight bookers like Travelocity or Expedia offer better prices than the airlines themselves.

On occasion, I have travelled to Madrid via Brussels using Virgin Express. The other ‘no frills’ player, Easy Jet, are currently offering some very good offers for flights to Madrid. Easy Jet flies out of Luton and Gatwick to Madrid.