All of our constitutional law takes the form of Acts of Parliament that can be amended or repealed with a 50%+1 vote in Parliament - unlike most countries where the constitution sits above the parliament and changing it requires a supermajority and/or a referendum. Boris had a majority so he could change the constitution. It’s a totally messed up system.
One reason British liberals as so passionate about internationalism and the European Union is that international treaties and EU law are some of the few mechanisms we have had for constraining executive overreach, since they sit outside and above Parliament’s remit. For example, even if Parliament were to repeal the Human Rights Act, Britain remains a party to the European Convention on Human Rights (which is why some Tories now talk about withdrawing from this too). Without international safeguards external to the UK, in theory all that stands between Britain and despotism is a simple majority vote in Parliament.
The latest possible date the election could have been is January 2025, but that was practically very unlikely as i) there is an extremely sharp generational divide in voting intentions (far sharper than in most Western democracies) and January would have meant the Tories having to get their elderly core voters to the polls in the middle of winter, and ii) a January vote would have meant a campaign running over Christmas, and everyone would have punished Sunak for that. The widespread expectation was for an autumn election.
It’s unclear why Sunak jumped earlier but likely a combination of various factors:
them being worried the economy will not get better by the autumn (so avoids going to the polls after a summer of bad economic news);
going early means their main opponents on the right (Reform) don’t have time to get their act together and select candidates in all seats (which they would have done by the autumn);
their flagship immigration policy is controversial and expensive, yet likely to have an underwhelming impact on illegal immigration levels, and they’ll look like complete idiots for centring an autumn election on a ‘stop the boats’ slogan if there’s another summer of small boat arrivals in the meantime; and
Sunak personally is fed up - he’s very much a political child of the far-right (an avowed Brexiter long before Boris Johnson or Liz Truss converted to the cause) yet the far-right of the Tory Party don’t see him as one of their own and have been constant thorns in his side throughout his leadership - he may just want out at this stage.