This is from a Missal from 1806.
Belgium under the late King used to have this insertion. It was the man who abdicated for one day instead of ratifying abortion.
Now, why would England get this in a Missal, when its Kings were heretics and known to be such?
Well, that is like asking why a Sedevacantist is using a Missal with "una cum Papa Nostro N." in the text : he doesn't read the line, but he hopes to be reading it as soon as there is a Catholic and admitted Pope around to read it about.
In 1806, under George III, this line was certainly omitted. One was probably hoping that George IV would publically convert, which would have implied the right and for a priest obeying that order from Rome even duty to read the line. It is possible that George III opposing Napoleon and abolishing slavery was taken as a hope of his converting himself, and George IV married in secret a Catholic woman, whom later he repudiated.
So, the answer was perhaps as simple as : Rome was hoping for a Catholic monarch in England.
It is also possible that some were reading "together with our King Henry Edward" - the last direct Stuart claimant actually was not just a Catholic, but a Catholic priest and Cardinal of the Roman Church. He is buried in the Vatican, in St Peter's Basilica.
This is not a sign that any Catholic priest was praying "una cum rege nostro Georgio" as long as George III had not converted.
Hans Georg Lundahl
PS, Fr. Cekada says:
"But in any event, as regards the priest’s altar Missal itself, the liturgical commentators are clear: The Missal of Pius V discontinued the mention of the king or civil rulers in the Te Igitur, and the practice was allowed only by way of privilege (as in Spain and Austria), where the ruler was a Catholic."
Obviously, an English King converting and risking his throne to be Catholic would have been worthy of that privilege. We can count on the privilege having been granted beforehand to automatically be valid from the day an English monarch was again Catholic - or of Stuarts, like Henry Edward, enjoying it./HGL