No Stupid Questions

No constitution, no hard checks and balances

I'm an American, but IIRC, the UK does have an unwritten constitution, one that incorporates all the landmark legislation over a millennium. That is to say, rather than a dedicated, singular document that "constitutes" the boundary of the law, the British look to their still-active laws to ascertain what core rights and responsibilities must exist, and extrapolate from there. If this sounds wishy-washy, it's remarkably no different to how the USA Constitution is interpreted, under the "living document" doctrine. That doctrine in American law simultaneous recognizes that: 1) the exact text of the constitutional provisions must be adhered to (this is a basic tenant of "rule of law", and 2) those provisions may extend to analogous situations. Right-wing conservatives over here attempt to ignore the second, adopting the so-called doctrine of "textualism" (which would only recognize strictly the first aspect) but this "doctrine" only seems to be cited out when it's convenient, and hand-waved away when it's not. Hardly a doctrinal approach.

As an example of what is universally understood as being part of the British constitution, see the Magna Carta. Many of its provisions might no longer be part of the formal British body of law, but were translated into formal statute law, with its lineage acknowledged when it comes up in civil rights litigation. The current status makes the Magna Carta more akin to the US Declaration of Independence, which formally grants or recognizes zero rights but is still important in American constitutional jurisprudence. In that sense, the Declaration of Independence is a part of the supplementary body of the American constitution.

As for checks and balances, since the UK adopts the notion of parliamentary supremacy -- and still does, even after the creation of the UK Supreme Court in the 21st Century -- the checks exist within the Westminster parliamentary system. As currently formulated, the UK Parliament is composed of a lower and upper house, with the former seating representatives of the people and the latter seating representatives of .... nobility? The church? I'll just say that the House of Lords represents the "establishment". Not like "deep state capital-E Establishment" but just the institutions at-large. In that sense, the check-and-balance is one where the populist will is anchored by institutional momentum.

Is this alright? Personally -- and again, I'm an American, not a UK citizen -- it does seem rather backwards that the PM can advise the Monarch to create and appoint more hereditary peers in the House of Lords, which could stack parliament against the interest of the citizenry. I think the existence of bicameral legislative bodies to be an anachronism, especially in the USA where both end up being population-based (because prior court rulings ruled that land-based representation was unconstitutional, except the US Senate). The Nebraska unicameral legislature shows what can be done when the law-making process (committees, 1st reading, 2nd reading, floor vote, etc...) is consolidated, where testimony doesn't have to be taken twice and citizens need only voice public comment at one committee.

But I digress.

No guarantee of stability, a new govt can repeal any of the previous govt’s laws

Yes, and no. The UK has a very rich tradition of inking out their party platforms, to the point that when a new government and party are in power, it's not at all a surprise what laws they will change. Indeed, it would have been obvious for months to years, since the minority party forms the "shadow government", which is basically a demo to the citizens about what the government would look like if they were in power. Note to fellow Americans: "shadow" in this case does not mean nefarious, but rather that each designated person from the minority will "shadow" the actual minister (eg Dept for Transport) and thus go on TV to give interviews about how the minority party would have done things differently. If a journalist needs to fill airtime with multiple points-of-view, going to the shadow minister on that topic is a quick way to get an opposing perspective.

The only question then, in terms of stability, is which party prevails after an election. In this sense, while there may not be absolute continuity, there is still practical continuity: businesses and individuals can make plans in advance when they learn what's in the platform of the minority party, can start actioning those plans if the party has a likelihood of winning an election, can brace for change if a close election is called, and ultimately be ready for when the new party takes power and implements their changes. It's a pragmatic approach: change is the only constant, so might as well give sufficient notice when things do change. I would offer Brexit as an example of managed chaos, since the lead-up to the election made it very clear that the UK might indeed fall out of the European Union. And indeed, they did, but only after 4-ish years of uncertainty and negotiations, which while extraordinarily tumultuous for the country, did not somehow devolve into wholesale governmental collapse or the sudden breakdown of civic life. So even in a near-worst case scenario that changed the very fabric of the UK's legal situation, it's still holding on. Not too shabby.

As for repealing "any" prior law, technically yes. But the institutional inertia is partially what blunts this power. Public advocacy organizations are -- to this American -- seemingly more transparent in their operations, and astroturfing is less an issue because of open-transparency when it comes to forming a legal company at Companies House. Likewise, the interests of businesses, the Church of England, the universities, workers unions, etc all find representation somewhere. So it's much harder than, say in the USA, to ignore whole segments of the population to make sweeping changes.