You can quite easily wrangle up a population of well over three hundred million (and approaching 400 million) from western europe alone for the WEU, which is plenty to compete with the USSR or UASR. I think the FBU would be pragmatic enough to accept the help of the rest of capitalist Europe, which if unified with the same foreign policy and a united economic structure; is pretty much an instant superpower overnight. And it seems that western Europe is in much better straits than it was in our timeline, or at least; Jello says it is.But those territories are not part of either france or britain proper - in the case of Africa they have been militarily coerced into submission. If the alt-WWII spills to much franco-british blood or destroy too much industrial infrastructure, they will have a hard time keeping control of it. Of course Britain can always count on Canada, Australia and NZ to help, but that would cause a power shift away from the FBU which is certainly not the intent.
And regarding South America the FBU has a sadistic choice: aid SA to industrialize itself and thus become an independent powerhouse or not invest and risk the impoverished population to be radicalized by communists.
And if they support the right factions in an empowered Brazil/Venezuela/Bolivia group they can get a loyal great or even superpower, which can also be done with Indonesia which also has a population roughly in the ballpark of the USSR and UASR (though lower by a hundred million or so). India remains the golden goose, and African and middle eastern resources are still likely to be competed over tooth and nail.
The combined spheres of influence of western europe's colonial empires and parts of South America is easily the single largest such empire in human history. You're talking about people who control half of the world's land area and more than half of its population (albeit much of it unindustrialized). That's a helluva lot.