I want to be clear about the argument you're making, as I think this is where our difference arises.
You think Trump, Berlusconi, and so on are selfish grifters but do not believe they are truly destined for greatness?
And you think Cameron, Rees-Mogg, Johnson and Truss are _not_ selfish grifters but are in it for the "greatness"?
I think you should do more research on both groups. Trump and Berlusconi (and many others I mentioned), seem to choose their actions based on self-interest fiscally and for their ego. They are over-confident in their ability to make good decisions because they feel they have been "chosen", and are "special". Their distaste for due process and the rule of law is evidence of this, I feel.
But so are the British leaders. Go do some digging, find out how they made their money and built their relationships in Parliament. Find out about the Bullingdon club, find out about Rees-Mogg's money, find out about the Institute of Economic Affairs, and more. You'll find every single one of the British leaders you named had the exact same profile of grifting and ego-driven self-belief that Berlusconi, Bolsonaro, Trump, W. Bush, and others have shown.
Your argument that there is something unique to Britain about this is weak to my mind. I can find identical behaviours of corruption, idiocy, grifting, ego and incompetence in recent history of every G20 country, every NATO member, every EU state, every democracy.
Writing this makes me feel cynical, but the good news is that there is plenty of evidence of competence and "greater good" style leaders around everywhere too - in Britain, as much as anywhere else.