Just stumbled upon this site, it is exactly the sort of thing I was looking for. You type in the airport you want to depart from, click the nearby link and then select a radius, then click select all. It will then search all of these airports, you can do the same for the return airport. Then select calendar of lowest fares, enter the length (you can say 7-14), then click search. It will show a calendar of flights and their respective prices.
[Only registered and activated users can see links. ]
There is also an Android and iPhone app called OnTheFly. Note that it doesn't seem to search Easyjet or Ryanair.
One example I found is flying from Dublin to LA and back is about £500 over Christmas, from London to LA is about £800 (even though it's the same flight and Dublin has a stop at London). Flights to Dublin on Ryanair are about £40 return.