Why was the government of britain an oligarchy rather than democracy

Firstly, I would say that for sure, during the medieval times, it was an oligarchy ruled by the aristocracy ie by a small minority of rich and priviledged nobles. Now, though nominally Britain is a democracy with elections, in fact very low numbers of people vote, like only 30% so in fact it tends to be the elites who get into power without a strong mandate from the people and implement policies as they see fit, most like as a neoliberal type government that defends the rich corporations against the interests of the broad masses of the people.