While I agree on some of your points, I think that you are confusing pre-war with post-war way of thinking.
In pre-war erurope
Britain was not part of pre-war Europe, or any other Europe.

(To be less facetious, "Europe" includes London and Moscow. I consider it too big to make useful generalisations about in social history or any other kind of history.)
a colonial mindset was still accepted as a reasonable way of thinking and the division between major (ruling) powers and minor (ruled) nations was not a taboo.
There's merit to the view that Britain refused to take small eastern European countries seriously (simply
vile things were said about them among backwoods conservatives fairly high in the hierarchy), but that didn't amount to a "colonial" mindset as seen in the Empahr. Orwell commented specifically on the hypocrisy of a British Conservatives who wants self-determination in Europe but not India. The idea that doing that sort of thing to white people was a bit far, old boy! went back to the Boer War, of course, and had caused quite the stir.
In this framework of mind, I do not see the image of a "British people willing to fight for the Czech democracy" fitting: some degree of elasticity wold appear much more probale to me in re-adjustments of borders and of spheres of influence.
I never said that Britain
would fight on behalf of the Czechs, only that we
could. Obviously some "elasticity" was palatable, since it didn't mean the end of Chamberlain's credibility and government; but as all I'm saying is that Britain would fight if called on, it seems to me that to disagree one would have to believe that between the autumns of 1938 and 1939, some dramatic change made the British people ready to fight for a cause still more obscure to them than the Czech one. Correct me if I'm wrong.
To say it with other words, the annection of sudetenland (which was done according to the will of most people in the region), and the eventual war on Czecho-Slovakia do not appear to me enough to trigger the (propaganda-boasted) elan necessary to wage an offensive war.
Now you
are ascribing European attitudes to British people, and dangerously. "Elan" is (of course) a French idea. I disagree with any broad strokes when it comes to how and why nations fight wars, but I
can say with certainty that
1930s Britain wasn't looking for "propaganda-boosted
elan"
, it was looking for something that would grind up all social and political ills and mix them together, so that although the anxieties of the decade could be resolved by having one's house bombed, eating a lot of turnips, constant moaning, and grim determination: things British society felt comfortable with.
In the case of the Sudetenland, the concept expressed for the rhineland applies again (their own home), and the refuse of czech government to negotiate on a german-inhabited area could presented as a legitimate casus belli.
This is actually expecting British people to keenly scan the diplomatic correspondance try to detect the truly guilty party. In the circumstances, that's not what people wanted to do. We wanted to smack ours hands together, roll up our sleeves, and say "Right, until we win, then".
Poland was run by a gang of colonels, and the immediate casus-belli was transparently German city of Danzig. A dictatorship menaced another dictatoship. Yet when it came to it, the British people not only did as I predict above; once our army had suffered a humiliating defeat and our principal ally had left the war, and with the USSR widely perceived as pro-German, American involvement not yet significant, and the talk of the town being how heroically we had managed to run away, the British people
chucked out a wavering government to bring in a hardline war-candidate.
You seem to be ignoring the actual events of OTL to repeat justifications of appeasement that were a bit highbrow for most people at the time.
(I agree on your arguments regarding intormation manipulation by the Chamberlain cabinet; on the other hand you must convene that there were some by the Churchill cabinet later, too).
Of course there was. "In wartime, truth is so precious that it must always be surrounded by a bodyguard of lies." - Churchill. But...
- War measures, war meaures. Defence of the realm. Doing our bit. Every country accepts that war is a differant matter; and historically, war with the Nazis had been pretty decisively vindicated.
- I am British and therefore regard everything with cynicism. I'm not arguing for or against wings of the Conservative party, I'm simply stating what I believe are facts about British public opinion.
Regarding the aggression to the Czech "core" (i.e. not-sudetenland), according to today's standards this would clearly be an break of the law of nations and a likely cause of war.
According to 1938-standards, however, that could be viewed as a re-adjustmet of influence spheres, such as italian annexion of Ethiopia or Albania, since what is a commonly accepted principle now (sovereignity and more-or-less equality between states) was not then (when a colonialist mentality was accepted).
Britain doesn't require referendums before war is declared: that's a decision for the government, and Chamberlain, who eventually did fight over Poland, would have done it over Czechia is the Germans insited on giving him no alternative. He did mobilise our forces and issue gasmasks.
Once war is declared, the attitude of 30s Britain isn't "Oh, the Germans were only adjusting their sphere of influence, let's make peace", it's "Well, war, I suppose. Bloody hell."
The midde-aged man in the Entent (expecially if he was a bit old-minded) was likely to have a train of thoughts like this:
A de facto alignment of several lesser nations (belgium holland) is something no one could deny: if germany is to be brought in the "concert of nation" again, some concessions were to be given to it: Czecho-slovakia is an acceprable sacrifice for the west; it is germany courtyard, after all
This imagines a creature known as an "Entente middle-man", ignoring the huge differences between the countries and within them. That quote accurately reflects the thoughts of many right-appeasers about
why they were offering Hitler free stuff; but once he's turned it down, that's no longer relevant. The arch-appeaser himself was willing to become a war-leader once a country he had been willing to sell out (Poland) was finally attacked openly.
Regarding the "tecnical" fact if the phony war was a military calculation or a miscalculation, this is a different subject on which we do not have the same opinion, but what I am trying to point out here is that the wage of a defensive war at the time was public-opinion induced, if not enforced
So what happened to British public opinion that changed it afterwards? And why did the French generals make offensive plans and even launch an abortive offensive? And why, most importantly, would a public opinion that supposedly forced a defensive strategy on Chamberlain grow impatient with the timidity of his military endeavours and replace him with a die-hard kill-Hitler-or-die-trying sell-our-souls-and-granny's-silverware war candidate?