Which of course we can do without being some sort of federation.
I dont see how. Modern warfare, particulary air defence and aircraft have become so expensive that smaller countries can hardly afford it. And the price is only rising. We cant do it now and even the war in Ukraine was motivation enough for the countries to the West of Poland to raise their defence spending accordingly. The UK, France and Germany could each field one division at best. The smaller countries could do even less.
assuming that the better off colonies
I don't need to assume anything. I can just point to the decisions those countries have made regarding Ukraine and Israel.
They look out for their own interests and their experience with British, French, Spanish tyranny has given them no reason to like us. If anything their experience with colonialism gives them views e.g. on the Palestinians that does not alogn with ours.
Very little chance of China
A lot of people thought the same about Russia a few years ago. The Chinese have a long memory. Their government reminds them about the time Europeans colonized them and forced them to buy Opium. They see how the Europeans oppose "reunification" with Taiwan. We have given them pretext for payback, even if it is just supporting out enemies.