The chief rabbi being frum would have asked a shaileh whether to accept or decline a Royal invitation.
It is highly fascinating whenever the royal guards with all their splendor and traditions do any ceremony, especially one as fancy as this. I also love the Changing of the Guards and also for the Queen’s birthday they do this whole fancy marching and dancing with full military bands etc. There are plenty of clips online to view. Do a search on Buckingham Palace.