I think part of the problem he has with characters is that to tell stories about such huge subjects they have to represent large amounts of people - he isn't really Belisle, Premier of Quebec, he's Quebec itself. Sands isn't the British PM, he's England. It's easier to work when you're dealing is such high concept plotting: "The US gives Canada an Ultimatum, but Canada isn't going to sit back and take it! And if the US invades - watch out, they're underestimating Canada". Everything is in such broad strokes that the character traits that he does come up with are rather perfunctory at best (Roussel is flawed - he likes to drink; Sands refuses to go to a hospital after being rescued because he possesses the requisite British stiff upper lip; Jessica Swift is elegant, slinky and superlative. Wait, those aren't actually character traits. But I really wanted to include one reference to the Secretary of State in this post).
Brian - I like your idea that Sands isn't clearly Labour or Conservative. Given when the book was written, either Harold Wilson or Edward Heath would have been in power - one of each. Likely he's a composite - he's only supposed to be England anyway.