According to Wikipedia, Gordon Brown got a first class honours degree and a PhD in history (Edinburgh), Rishi Sunak a first in politics, philosophy and economics (Oxford). The Wikipedia page for Liz Truss says she was also at Oxford but doesn't say what she studied or what kind of degree she got. However, the Merton College site has a
page about her saying she studied PPE too but also not saying what kind of degree she got. So probably unlikely to be a first.
Wikipedia says Margaret Thatcher got a second-class degree in chemistry and Tony Blair a second-class honours degree in law, both from Oxford.
Thatcher was apparently president of the Oxford University Conservative Association and Truss president of the Oxford University Liberal Democrats.
I wonder if the fact that Thatcher, Blair and Brown didn't study subjects that suggested they had political ambitions (I gather that PPE is a subject often studied by aspiring politicians) made them better leaders.