I believe I see things clearly and I see opposite of what you say. Ambassador is an extension of government. He must say what government says. If Trump wants to have Farage close to him he should give him some White House job, but the ambassador from Britain must be someone who speaks for Theresa May. Otherwise he is an ambassador of nothing.
Hence we have definitive evidence that Prime Minister Theresa May is not serious about Brexit since she is unwilling to appoint its most powerful spokesman and promoter as her ambassador to the USA.