If I understand the problem correctly, it isn't as hard as it appears.
Let the price of the Item be P, in in the year 2009. P E = P £ in 2009
Then the price of the Item, in Euros, in the year 2011, is (.90)P
As you've noted, the decrease in terms of exchange is 10%, so the P in 2011 converted into pounds is (.90)(.90)P = (.81)P
In summary, the price of the item went from P£ to (.81)P £. Hence the net decrease (in terms of £) is 19%