The antithesis of populism can be found in Singapore. It is a democracy, because there are elections and society is supposedly based on meritocracy rather than a class system based on nobility. However, once people have cast their vote, they get little say and the government is autocratic. People have effectively made a decision about who governs them, but not how they are governed.
Populism seeks to do the opposite. The voting system in the UK means we choose our representatives from a limited list and then are supposed to trust them to do their best to represent their constituents. We know their stance on various issues (hopefully) and accept them as a complete package, so we might prefer somebody who supports gay marriage, although we don't like his/her stance on nationalisation.
Populism seeks to make MPs delegates, so they would have to go back to their constituents and seek their views on single issues and vote accordingly in debates, even if they thought that their constituents' views wouldn't be good for the country as a whole and didn't match their own beliefs.
When people lose confidence that experts and leaders have a better overview than they do, they will seek to make their voices heard, as happened with the referendum. We are now seeing a government reduced to the role of servants, as they attempt to negotiate something which many of them don't believe is good. Effectively, populists override the role of Parliament. Populism can result either from anarchy or be exploited by autocrats wishing to seize power without going through political processes. It's a breakdown in current processes and can descend into mob rule, a situation which has historically been exploited by dictators.