How many stamps to mail letter from TN US to UK?

Answer 1

According to the US Post Office postage calculator (link attached to this answer) the current cost for the most basic postage, first class letter to from the US to the UK, is 94 cents. If you don't want to go to the post office to get the exact postage you need, three first class stamps would do it. You'd be overpaying, though.

International Postal Union treaties used to require that postage have the cost printed on it to be honored for international mail. If this were still true, the "Forever" Stamp wouldn't be honored for international mail. However, the US Post Office stated in an online press release that the Forever Stamp is fine for international postage. You do, however, still need to add additional postage equal to the difference between the current US First Class letter rate and the higher International rate.