Just to clarify. Due to parliment own rules. That they can change if enough agree. They can only call a vote of no confidence a few times a year. Because they have to wait after the last vote.

So while quickly replaced at any time is technically correct. As we saw during Mays gov. Even a weak PM Majority wise. Also leads to a weak parliment agreement wise. So things always become more complex then the rules indicate.

EDIT: I am going to add the following additional, info because I personally think it is important rather then it is relevent to your question. It is more political opinion then fact.

When I say weak PM and weak parliment. It tends to support the claims of many First Past The Post supporters. I am not one of these.

They tend to beleave fptp leads to stronger governments that can make decisions without rellying on multi party agreement. IE they feel it creates less situations where we have weak divided parliments and resulting PMs due to this.

While the few examples like Mays government make that ideal hard to argue with factually. Weather it is actually a benifit is much more questionable.

Any look at voting percentages supporting the government parties over the last 50 years of our history. (I’m 53 to explain the rather odd choice of timeline).

Shows that we historically have more strong governments representing well under 50% of voters then overs. And far to often as low as the 30s%. It is pretty hard to make any claim that a government that only represents 1/3 of voters. Should be strong enough to not need to compromise with other parties.

But again my personal opinion. I think it is much worse then that. During the Brexit cluster faild breeding exercise. I came to the following conclusion.

Generation of first past the post strong governments. Have damaged our nations understanding of the need to compromise.

Post the referendum vote we had just over half the population. Thinking they had won exactly the brexit they were promised. Dispite the fact multiple self contradictory ideals were promised.

But worse during the debates with the EU. We had multiple people suggesting we needed a second vote to decide what type of brexit was wanted. Followed by hard brexiters openly stating. But that would devide the brexit vote. Literally stating they think they have a right to the votes of people wanting contradictory version of their brexit.

This really cluster bombed the ability for parliment to come to any agreement with the EU.

The other EU nation. Who nearly all have more proportional governments could clearly see more compromised solutions that fitted the majority of voters more closely then the deals the government was asking for.

But parliment was week. As well as the PM. But still unwilling to form any compromise on a way forward. EU nations rarely see anything like this. Because most of there governments are built from multiple parties haveing to come to some agreement on governance. While our government agree to ideas. That they could not even get their own parliamentary members to back. Let alone most of parliment.

Then the solutions. VONC followed by an election. Where a clown promised a version of brexit most knew was not possible with the EU. But approx 45% of the UK supported it.

Giving the clown enough strength to take his raw potato and tinfoil oven ready brexit. And agree to a lettuce sandwitch. Dispite the face even his 45% in no way wanted that.