On the news last night they said only current heads of state were invited, so Carter, Bush and Obama were not going. I could see if for no other reason, security would want to limit the number of people present. In 70 years of her reign, it would make an awfully big crowd if all the previous heads of state showed up.

One of the news shows had a clip of him with her inspecting the Guards (I think)
It was the proverbial "picture worth a thousand words". There she was, her usual dignified self...and them him, with the trademark scowl. This country should be so ashamed.

He couldn't go anyway as didn't the FBI grab his passports?

Who in their right mind would want that unspeakable at their mother's funeral?

I'm sure the Poms told each country, "You are allowed xx people." It will then be up to them who comes so in the US's case, up to team Biden.
Some exception for commonwealth countries eg Victoria Cross recipients, the highest military honour awarded by the British monarchy, are invited.

NB I was in the Australian Army 86-89 and swore allegiance not to Australia, but to "Elizabeth reigning, her heirs and successors.". Still the same now I believe. Thought it sucked at the time, still do.
Our officers are commissioned officers ie commissioned by the Queen (now King). When we salute, we salute the commission not the person. In plain speak, we salute the bird shit on their shoulders.