By Anne Marie Roantree and James Pomfret HONG KONG (Reuters) – Barristers from a British law firm who are representing jailed Hong Kong tycoon and democracy campaigner Jimmy Lai said on Thursday they had received anonymous emails warning them against travelling to the city to defend him. Lai is among the most prominent people to be charged under a national security law that Beijing imposed on Hong Kong in June 2020 to punish terrorism, collusion with foreign forces, subversion and secession with possible life imprisonment. Critics of the law say authorities are using it to stifle dissent, a ch…