The letter’s progress is a source of media fascination. In an age of instant communications, the delivery mechanism is reassuringly archaic, particularly as the contents are well-known - those contents could have been sent instantly in an e-mail, the original document first having been signed and scanned, which is good enough for a lot of legal documents these days.
But not this time. The letter was instead borne by special messenger to the white cliffs of Dover and thence across the channel that divides us to continue its journey to the capital of the European project.
There it was passed into the hands of our man in Brussels, Sir Tim Barrow, who put it in a black briefcase and prepared to execute the one job demanded of a top diplomat - deliver.
While the letter was somewhere along the way, I decided to walk the dog, mostly to avoid listening to television and radio news bulletins speculating about where the letter might be.
But first, on this very symbolic day, I chose a breakfast of beaked beans on toast as very symbolic of the things that divide we Brits from the rest of Europe.
Then, resisting the temptation to wear my Union Jack bowler hat, I took Mikey the dog through the streets while I pondered yet again how we, the British nation, arrived at this state.
Mikey doesn’t care. He was abandoned in Sicily, brought back to live with us, and he is going through a barky phase. Other dog-owners tell me that it’s not a phase: he is a dog, therefore he barks. It’s a bit like being Nigel Farage, which is why Mikey’s confrontations with other dogs in the street are now called Barks-it.
Mikey triggers Barks-it negotiations and pursues them until the opposing pooch either gets bored and goes away, or gets aggressive and refuses to budge. Which brings us neatly back to the confrontation about to begin.
By noon I am outside European Commission headquarters, where tourists with cameras are taking pictures of film crews with cameras, who are filming people saying things about what might happen next. This includes me.
While this is going on, Sir Tim’s chauffeur is putting the final shine (I imagine) on the lustrous black paintwork of the brand new embassy Jaguar XJ long wheelbase limousine which only rolled off the delivery truck outside the UK’s delegation office to the EU five days ago.
This is what you call keeping up appearances: for a long time. the official UK limo in Brussels was a German car we shall call a B*W.
Then Sir Tim’s predecessor as head of delegation, Sir Kim, who likes his cars and his nation, pressed the Foreign Office for a thoroughly British machine to wave the flag for the nation in the EU capital.
This was a few years ago, before Brexit was a word but after Jag became Indian-owned, albeit still totally built in the Untied Kingdom.
Now, possibly with calculated timing, a replacement has been decreed, a fast new car as a sign, paradoxically, that the Brits are not going anywhere, even after we’ve left.
One of the Jag’s very first tasks, therefore, was to carry the dashing British plenipotentiary round the corner from his office to deliver the aforementioned letter to the President of the European Council, Donald Tusk.
Sir Tim could have walked there quicker, as the car had to navigate a roundabout, which Sir Tim on foot would not. But he could hardly walk through the streets on such a mission while being pursued by reporters and film crews. Now could he?
Thus Sir Tim, who is doing for beards and three-piece suits what David Beckham has done for tattoos, slid into the right-hand back seat of the Jag, clutching the briefcase containing the letter, and went on a very very short journey for a man, but an exceedingly long one for (British) mankind.