All the UK PM's who went to English universites went to either Oxford or Cambridge.
I don't think that is a surprise. I'd expect PMs are more likely to be university educated than not, and they are considered some of the best universities in the world.
That's not a meritocracy. To work a meritocracy has to have mechanisms which ensure that each generation can achieve their potential, so an oligarchy cannot take root. There will never be a perfect meritocracy but many countries do it a lot better.
It's good to have mechanisms to ensure that all people can achieve their potential. But all systems need mechanisms to make them work and have potential failings associated with them, including meritocracy.
A lesser one is how to prevent the children of the wealthy and powerful from starting off ahead to the degree that it means they will almost inevitably remain on top. You can do that somewhat for material things, but it's less clear that you can compensate for other characteristics. If over time you find that children share the merits off their parents, you can soon easily have a class system, even if there is not a huge wealth divide.
But what's more concerning IMO is a tendency for people to think merit makes them deserving, or better or whatever. It's the exact mirror of people who think belonging to a particular social class makes them better, except they are even more likely to believe they really deserve their advantages.
Meritocracy also tends to imply a scale of importance, where certain types of work are better, for those with more merit, compared to others. Aside from the obvious effects it can leach very talented people out of certain types of work that are important but less valued.