Look - take the political divisions off the table and look at some basics...
If you are a strategist/visionary/mega-leader, you need a cabinet that can provide the details and implement. (wit Churchill) and, in theory, Boris (1st class from Thatch's alma mater).
If you are a details person, (like Thatch - who held a 2.1), then you need the strategists somewhere in Cabinet - such as Patten, Clarke etc.
If you are a Crisis person, (like Blair - though he bungled the Iraqi War when the evidence was there), you need a good balance - Blair was no strategist but brilliant at PR. He had a 1st from Oxford.
If you are Jeremy Corbyn - you need MI5 or 6 to take you out as there was no hope as to Cabinet as he didn't understand and was just so blinkered. As to his intellectual qualifications, 2 Es at A-level which, to me, sums it up - no innate capacity to absorb, listen, synthesise, conclude and then provide the leadership. There are those who can do this - wit Richard Branson.....