When I first went to the USA four years ago I was told I needed an ESTA. Not having been abroad for some years I assumed this would be something stuck in, or stamped on, my passport, or a document I needed to show border officials on arrival in the USA. However I have been to the USA a number of times since and have never been asked to show my ESTA document. I therefore assume when border officials scan passports they can see on their computer screen whether or not someone has a valid ESTA. (I know there is a clue in the name ESTA.) What I would like to know is, if someone attempted to travel to the USA without a valid ESTA would they be stopped at Heathrow, or other UK departure airport, from boarding the plane or would they have to wait until they arrived in the USA to find their ESTA was invalid?
