Indeed, referendums cannot be binding in the UK, because that would be taking sovereignty away from parliament.

There have only ever been 3 national referendums in Britain. One to join the EU, one to leave the EU, and one over Alternative Vote. They used to be considered unconstitutional, a tool for demagogues. I would be happy to ditch them forever.

and if Stay wins, of course the result will be treated as Final?

If a future referendum indicated that there was no longer support for leaving the EU, then yes, the government would not be in a position where it needed to take further action - call that "treating it as final" if you wish. Of course, in the future it could be reevaluated, using the same approach that led to the 2016 referendum to leave the EU.

Heck, we have confirmation dialogs when you close a word document - I don't understand the opposition to having an "are you sure you want to quit?" for brexit.

Brexit is a pretty major decision, and it's not unreasonable to me to require that the majority that voted for it is in fact stable and consistent via holding a second vote. If the second vote does not indicate that a majority supports leaving, that seems like an indication that it was a fantastic idea to hold a second referendum.

As much as the 1975 referendum to join was final!

The British Empire was final, as were the other colonial powers; the Soviet Union was eternal, the Third Reich was supposed to last a 1000 years. Meanwhile, shift happens.