Had a hunt around the web last weekend for "Taux de change " & "Taux d'echange" and couldn't find anything concrete on the Gouv.fr or impots sites re a figure for exchange to be used on the tax return, maybe I didn't look in the right place ? I did find on another site ( cant remember which one though as I looked at loads ) a figure & do remember that the Banque de France was mentioned so I've gone with that. The rate quoted was € 1.2074 to the £. I dont think it can be a million miles off but it's up to you if you use this figure or do further research ,just in case it's wrong .
( If they bother to check it anyway