Why fly at all? Why not hold meetings over the internet like other working people? Or organize the conference in Bradford or Blackpool, instead of some tropical paradise, and see how many people are still motivated to fly in?
Speaking as someone who flies for high-value meetings, I will beat the guy literally phoning it in about three times out of four solely because I expended the effort to meet in person. Partly because it's a social gesture, showing I'm willing to expend time and resources for the person [1]. Partly because we're human beings who connect better in person than virtually.
When the stakes are international relations, the CO2 impact of the flights is peanuts.
And what are the stakes in practice?
Meetings of the G7 probably have a real impact on international relations. But in this comparison, the UN is cargo cult politics (they pretend really hard to be doing it) and Guterres' travel to Tonga a barely masked vacation at someone's else's dime.