UK retailer Kingfisher had its rating whacked down to 'sell' from 'hold' by Societe Generale on Friday after it cut the B&Q owner's forecasts due to currency swings and took a dim view of the company's outlook.The French investment bank makes its downgrade on Kingfisher, who owns UK retailer B&Q, after last week's full-year results."We are reducing our pretax profit estimate for the current fiscal year ending January 2016 by around 6% to £656m from £697m," said SocGen.It added that the euro has weakened since November 2014, leading to a 10% cut in the bank's estimate for Kingfisher's France trading profit."The weakening of both the Polish zloty and the Russian rouble against the British pound also contributed to a lesser extent to the reduction in our estimates," SocGen's analysts added."Kingfisher is more or less mature in our view. Medium-term earnings growth relies on successful restructuring and synergy benefits, which we expect will take time to achieve," said SocGen.The bank thinks that organisational change could bring execution risks for the company, while the UK home improvement market could weaken from the healthy levels enjoyed between 2014 and 2015.