This isn't even about patriotism, or jingoism or Britishness or any of those subjective, nebulous, contested terms.
It's about being PM and having obligations and duties to perform. Some are sombre, some are lengthy, some are probably really boring, some may not be in line with one's own personal ideologies (I'm sure thta happens loads) but as PM you bloody well go to them , stay as long as it takes, shake the hands, do the chit chat and nodding.
It's not only Royals that are supposed to have this sense of duty and service.
Hs comment on young people and national service are lost on no one today.