However, I think the key reason why Conservatives and Labour are so entrenched is that people make their voting habits a part of their identity. I had a number of face to face conversations about politics with people born and raised in the UK. Every single one agreed with me about many stupid things the back then conservative govt pushed (the idea to ban encryption and more). And every single one of them said they will continue voting Conservative. Why? Because this is who they are. It's a part of their family identity (being quite well off financially, having expensive education etc). And they only see two choices, with the other being much worse.
This is how democracies die. They even agreed with this being far from optimal, but they see no other option.