Chris Mason: Vivid interactions between Crown and parliament

Outside Buckingham Palace, there is a lovely atmosphere.

A trip here wouldn't be for everyone, but for those who have decided it is for them, there is a thoughtful, quiet, polite, reflective mood.

Some are carrying flowers.

A big white flag with the Queen's image printed in the middle is being waved in front of me.

Drifting overhead, a silver, heart-shaped balloon, liberated from its owner, intentionally or otherwise.

Those in the crowd and those of us here observing them are witnessing first-hand the British constitution, florid in its decoration and freighted with history, exposed to a global sunlight like never before.

The interactions between what the 19th century writer Walter Bagehot described as the "dignified" and "efficient" elements of a constitution - illustrated in the UK's model of governance.

Dignified referring to the monarchy, efficient to parliament.

The two, of course, interact - often in private, occasionally in public, such as at the State Opening of Parliament, when the monarch travels to Westminster to read out the government's planned new laws in the coming year.

For years, we called it the Queen's Speech.

Next time, it'll be the King's Speech.

But those interactions between the Crown and parliament are vivid right now.

Carved into the exterior stonework of the Palace of Westminster - the House of Commons and House of Lords - a king past, as a new one ascends.

The statue is of Henry II.

Inside, MPs have sworn their allegiance to King Charles III.

Some seeking the help of God. Others the law.

In English. And, for a few, in Welsh.

At Buckingham Palace, audiences with the King for the new prime minister, her new cabinet and opposition party leaders.

The magnitude of these moments isn't lost on any of those caught up in them.

Just as it isn't on the crowds outside, as the world watches.

In the coming days, the new head of state, the King, will visit Edinburgh, Belfast and Cardiff.

The new head of government, the prime minister, will do too.

Each introducing themselves to the country, in their new roles, at the same time.

I'm told Liz Truss will attend a church service in each city; her role won't be that prominent.

But it strikes me at a time when there are strains on the architecture of the UK, those who argue nations within it might choose to go their own way one day, these visits carry a political value.

The Prime Minister of the United Kingdom of Great Britain and Northern Ireland able to visibly illustrate that full job title so early in her tenure.

Before that, though, the arrival of the King here at Westminster on Monday, to address MPs and members of the House of Lords.

Another moment that will be quite a moment.