I did not receive an e-document that states "UK ETA" on it. All I have is the e-mail I got from the UK Home Office.
My biggest concern is not so much whether I can get through the UK Border Control, but whether Delta will let me board the flight from the USA; in other words, how does Delta satisfy itself that my UK ETA has been married to my US passport? Has Delta instructed its counter agents at the initial departure airport to honor a printed copy of an e-mail from the UK Home Office that the ETA has been granted should there be some discrepancy in Delta's on-line systems?