I think the story is much, much more complicated. For starters, it feels like different (albeit related) notions are being conflated: genuine vs feigned stupidi

I think the story is much, much more complicated. For starters, it feels like different (albeit related) notions are being conflated: genuine vs feigned stupidity, and public election vs internal promotion. For example, it's unclear why Boris Johnson would feign stupidity rather than simply feign confidence, of which he has ample anyway. David Cameron has achieved political success by appearing and being both shrewd and confident. As for Cameron's choice of cabinet, a more personally satisfying explanation for his cabinet choices is that they are loyal to the point of taking the bullet for his less popular policies. I don't think any of them are idiots or even appear idiotic to the general public, despite their policies being vulnerable to criticism.