I ask my local tax office every year. They have a very thick book which, among other things, contains the official rates for world currencies. They give me the December rate and tell me to use that. It might be different if you're running a business as I seem to remember you are technically supposed to use the rate which applied at the time of each individual transaction. Which is a nightmare.
The "Connexion" received this week is suggesting a rate of 1.1775 euro to the pound as an average for 2013. Apparently this rate was recommended by the Finance Ministry & the Banque de France.