Until midnight tonight (24 Nov)
www.Ryanair.com are offering free flights to Spain (Valladolid, Murcia, Girona, Jerez) from the Uk (mostly from Stansted)
Of course
a) most flights will incur some taxes
b) If you try and fly at a popular time (eg weekends, holiday times) the prices will be higher.
Even so, if you are planning a winter trip to España, this site may be worth looking at. I just wish I had seen this offer earlier this weekend to let UK based readers of this site know.
PS I flew a few years ago from Glasgow in Scotland to Dublin for around 10 pounds. Their offers are genuine.