By Nina Chestney LONDON (Reuters) - Britain should reduce its reliance on corn ethanol for renewable fuels and encourage the use of cheaper and more sustainable bio fuels such as used cooking oil, a report by a UK think tank said on Tuesday. It said the government should consider modifying its Renewable Transport Fuel Obligation (RTFO), which currently requires that 5 percent of Britain's fuel supply by volume is made up of renewable fuels by 2013-14. The RTFO should instead have an energy content target or set specific targets for ethanol and biodiesel, Chatham House said in the report. ...