London can be unbelievably cheap to fly from, though usually not LHR.
STN, LGW and LTN are major LCC hubs (Easyjet and Ryanair) and can fly you almost anywhere in Europe for peanuts, even with taxes. I flew to MAD from LGW with Easyjet in September 07 for £45 return (about the price of a nice meal) and WAW with Wizzair (not recommended) from LTN for less than £50 return during the Xmas period.
The LCC market out of London is by far the biggest in Europe and the competition between the carriers can be intense at all times of the year (Easyjet v Ryanair is one of those classic business bun fights) - though you can also be suprised by how expensive LCCs can be at busy times of the year or if you reserve at the last minute.