That depends -are you flying into London and immediately hopping to Spain? Or spending time in London first?
If you're spending time in London, then it's easy enough to get to one of the smaller airports (i.e. Stansted, Luton) and take RyanAir or EasyJet to Spain on the cheap.
If you're flying into Heathrow and you want to make the jump immediately, you can try BMI, who have a number of direct flights into Spain on the cheap (though not to every city...) British Airways and Iberia have the occasional good Heathrow-Spain deal, as well.