There are a number of factors involved as I see it. Unfortunately, without a system of proportional representation, you end up stuck with two dominant parties and elections end up being about voting for the least awful of the pair (from one's perspective) rather than any positive endorsement.
And I think because of this two party dominance, the political rhetoric is very adversarial and each side has to move farther to the right or left to oppose the other. Most people want sensible, balanced, centrist policies, and cooperation. Yet it seems neither party can even bear to agree with the other. Electoral promises are about undoing what the other side has implemented, so it's just this back and forth without any real consensus taking place. Only broad consensus can help move things forward.
The UK, in general, is a relatively socially conservative society, but it also can be quite open and liberal in its 'live and let live' approach. A lot of progressive types tend to sneer and look down on those who don't share their values completely. They mock and denigrate those they disagree with, calling them racist and fascist. People have had genuine concerns for years on issues like immigration, but no discussion is allowed on such matters in progressive circles. It is absolutely 100% perfect, with no downsides or drawbacks. This is obviously not true because everything has its advantages and disadvantages, yet one side refuses to engage with the concerns of the general public and insults them, but also expects the same general public to vote for them! It's really hypocritical to denigrate someone and then express shock and outrage that they've decided to vote for the other party. Apparently the best way to deal with this outcome is to denigrate them even further, because next time they will really feel like voting for you.
This is a well-respected report on why many voters turned their back on the Labour party. I have not seen any genuine engagement from progressive types with the issues raised in the full report. It's definitely worth reading.
lordashcroftpolls.com/2020/02/diagnosis-of-defeat-labours-turn-to-smell-the-coffee/