Invites were sent to the Heads of State. So where the Head of State is none elected The King/Queen/President would be the representative. All heads of European Monarchy attended as well as numerous others. Countries, like Ireland, Israel, Germany, South Korea and Italy, sent their Presidents. Where the President is also the political leader then they will have been invited eg USA, France, Turkey, Brazil attended. China sent its Vice President as did a number of other countries. The wife of the Ukrainian President attended on his behalf. I believe a had a minor matter to deal with at the time. Owing to strained diplomatic relations the ambassadors of Iran, Nicaragua and North Korea attended. The following countries were not invited Russia, Belarus, Myanmar, Syria, Venezuela and Afghanistan. Representatives from the UN, the EU and the Vatican city also attended.
Obviously there were circumstances where the ambassadors of some countries had to attend for whatever reason.
150 countries were represented. Only the Presidents of the USA and Israel were allowed to use their own cars.
The Thai community in the UK is 'not happy', by their county's representation.