By Andrew MacAskill and Sinead Cruise LONDON (Reuters) – Banks in Britain have warned the government that a plan to ban all Russians from holding more than 50,000 pounds ($65,430) in their accounts is illegal and unworkable, five sources said. When Russia invaded Ukraine the government said that “all Russians” would be banned from holding large deposits at UK banks, with Prime Minister Boris Johnson saying it was part of a strategy aimed at isolating Moscow. There are currently more than 70,000 Russian-born nationals living in Britain, government estimates show, and London has long been one of…