The anon the person sent saying you can’t use a title unless you’re a UK citizen is wrong. The Duchess of Windsor never became a citizen of the UK. She always maintained her American citizenship for obvious reasons (the UK would never give her citizenship). And yet she was allowed to call herself the Duchess of Windsor by the King (and later the Queen). The UK doesn’t give titles like Dame or Sir to citizens of the US but if they marry into the family, it’s different.