The definition of "middle class" in the UK is different... Unintuitively middle class denotes a social standing well into the top decile - members of the middle class would be typically educated with a graduate or professional degree, top-5% salary, multiple properties, children in private schools. (Whereas in the US, any white collar person - so basically half the population - is "middle class".)

Yes, the UK middle class is over-represented at Oxbridge, but the definition is so narrow that they are outnumbered by the lower classes - state school kids and other riff-raff constitute two-thirds of Oxbridge classes, especially outside the upper-class signal subjects like Classics.

>top-5% salary

Well that's hardly the middle is it.

It's not the median but not intended to be - it is in the middle between the upper class (<1%) and the lower class, which used to be more than 90%; it's a description of all the professional jobs (doctors, lawyers, merchants) which are neither serf/farmer/servant/laborer nor the upper class.