Royal Protocol Specialist
Some guests are always invited:
- all Royal Highnesses
- the Prime Minister, Foreign Minister, leaders of the House of Lord and House of Commons, and the leader of the opposition
- the Archbishop of Canterbury and Leader of the Supreme Court.
- the Doyen of the Diplomatic Corps and Ambassadors from a couple of neighbouring countries to that visiting.
- senior courtiers, the Great officers of state and those in-waiting on the royals.
- previous British ambassadors to the visiting country are usually invited.
Within each ´category´ there might be a need to reduce the number if the visiting party is very large, and I assume the easiest for BP is to reduce the number of courtiers.
Other than that, the guests are usually people with some kind of connection to the visiting country, and sometimes there are people especially invited as a courtesy to the visitors.
All guests are invited with their spouse.
The Marchioness of Cholmondeley was present because her husband is a Great Officer of State and usually always invited.