By Sinead Cruise and Iain Withers LONDON (Reuters) – The British government is extending a trading plan to help sell down the taxpayer’s stake in NatWest Group by another 12 months, as chills in the global economy continue to weigh on the share prices of Britain’s biggest banks. The finance ministry said on Wednesday it will only dispose of its remaining 48.5% shareholding in NatWest “when it represents value for money to do so and market conditions allow.” The government has stepped up efforts to return NatWest to private hands in recent years, after bailing out the lender formerly known as R…