It's complex. There's nothing really to stop a British prime minister going full dictator if they want, as long as they can get Parliament to pass an Enabling Act. And that's not an entirely fanciful proposition: they have the support of a majority of the House of Commons pretty much by definition and the Lords have very little power in practice.

Our constitution is built on the so-called 'good chap theory of governance', which is that power is in the hands of people 'like us', and therefore reasonable. If the PM loses a vote of no confidence he's obliged to resign. What makes him? Nothing. But that's the convention and it's been followed until now.

It's also a constitutional convention to resign from ministerial office if caught lying to Parliament. (Yes, really!) That one, though, looks defunct: Boris Johnson is a serial liar, and has managed to get himself referred to the Privileges Committee of the Commons for lying over his lockdown breaches despite his majority in the Commons. Will he resign? Will he hell. He doesn't want to, and all the Commons can do is bring the whole government down (and even then only if he is prepared to resign then). He's made it clear he doesn't care, and the Conservative Party in the Commons has made clear they're not willing to face an election over it, so that's that.

A similar thing happened over the Prorogation of the House just before Brexit. That was fairly straightforwardly a contempt of Parliament, and the advice to prorogue was found to be unlawful by the Supreme Court. But nothing of consequence happened to those in charge as a result.

I'm grateful our political culture isn't quite as buggered as that of the US at this point, but I'd be grateful for slightly stronger guardrails against misconduct than simply hoping it doesn't happen.