When I flew EasyJet London-Madrid it was out of London (Luton) which, as I understood was Easyjet's London 'Hub.' [Cool]
Eddie, Easyjet started 10 years ago from London Luton and became by far the biggest airline there. The owners then decided to develop Luton and build a new departures building, and charge all airlines more to use the airport. Note, Luton is not owned by BAA, unlike Stanstead, Gatwick and Heathrow.
Easyjet were against this policy because it contradicted what they were trying to achieve in bringing down European air fares, so they launched a campaign against it which included developing routes out of both Stanstead and Gatwick. While Luton does still have the majority of Easyjet flights Gatwick is now a close second. There ar in the Summer three scheduled Easyjet flights a day to Madrid from Gatwick, in addition to the two from Luton. I suggested Gatwick because Heathrow to Gatwick is marginally easier than Heathrow to Luton.
Easyjet have modified their web site now btw, so when you search for flights from London, you may either specify Luton or Gatwick, or search all of them. There can be significant price differences, especially if one flight has been booked by a group, so it is always worth checking all routes.
Hope you find that useful.