I think the British have to give what remains of Czechoslovakia a Polish-like guarantee in say December 1938, after a few by-elections have all gone to the Labour or Liberals, with the Conservative candidates being branded as "friends of Hitler". The Chamberlain government remains in the saddle, but decides a very clear warning has to be given to Hitler.
Naturally, Hitler still needs the Czech factories and the gold stored in the central bank. So he goes ahead.
See post #25. In order to create a basis for war over Bohemia in May 1939, the PoD has to be in say December 1938, with the British signing a guarantee, and then probably a full alliance, with Czechoslovakia, and an involvement of the Czechs in the planning. Hopefully the Soviets get recruited too. The bottom line is that the Czech government turns down Hitler's meeting invitaiton (ultimatum would be the better word). No agreement. Germany has to declare war or invade in the face of live-fire opposition.
I see your concept, stated twice above, that a precondition of the west declaring war over the occupation of Bohemia in spring 1939 is the British govt drawing a line in Dec 1938 after Munich, motivated by domestic political pressure (You remarked on losing some by=elections to Labor at that time. I don't know if there were any OTL, or who won them)
I think your concept is the most natural one in terms of setting up the international diplomatic pretext for a casus belli/DoW.
Declaring war in March after warning Germany that moving another inch means war the previous December makes more apparent diplomatic sense than declaring war or issuing an ultimatum out of the blue right after the occupation of Bohemia without any prior warning save the nominal guarantee of rump Czechoslovakia at the time of Munich.
....on the other hand...
...I would argue that avoiding explicit threats in fall 1938 but then suddenly demanding German retreat on pain of war in spring 1939 actually makes *more* sense from a British *domestic politics* point of view.
I base this on analysis of British opinion, and British cabinet discussions, described in RAC Parker's "Chamberlain and Appeasement".
In the immediate sense of relief from impending doom after Munich, it would be nigh impossible to shake Chamberlain from his confidence that the appeasement tactics worked and threats would be counterproductives.
It would have seemed too pessimistic right after Munich to threaten Germany about further expansion and do staff stalks to render guarantees effective immediately after Chamberlain's homecoming.
However, the sense of anger and betrayal became overwhelming after the occupation of Prague and many conservatives and most MPs thought Chamberlain's reaction was far too weak.
*This* is the time when more Cabinet or conservative party members were advising war or threatening to resign or break with Chamberlain.
Chamberlain could have been very vulnerable to a complete loss of faith in his own previous policy, or an intra-conservative coup, or a fatal or incapacitating heart attack or stroke in the immediate aftermath of the occupation of Bohemia, and any of these events would have opened the door to the British cabinet deciding to issue an ultimatum for the Germans to evacuate Bohemia, or else war.
I don't have any descriptions of the Soviet/Allied negotiations in front of me. A close look at what the Soviet representatives proposed would be a indication. I have some vague memories of inquiries; about loans, Britains banks had capital to lend & Germany had none; removal of obstacles to services of industry, probably petroleum; removal of obstacles for Soviet exports; easier purchase of industrial items from Britain and France. I suspect expanding capitol investment via loans and export/import sales looked more flexible, efficient, and attractive than the barter system with Germany. Most descriptions have vague remarks about the military side of the discussions, & nothing about the other aspects.
These all seem pretty harmless from an Anglo-French point of view.
And then Hitler would've waved the April 14th accord, signed by Czech president Emil Hácha, in which the latter had laid the fate of the Czech people into the hands of the Führer of the Greater German Empire, in front of everyone's nose. That said signiture had only been affixed by the very outspoken threat that, shouldn't it be, the Luftwaffe would start bombing Prague to the ground at 04:00 hours next day and after Hitler's quack had sufficiently resuscitated the poor old man after his circulatory collapse upon hearing this and reportedly being chased by Göring around a billiard table, to do so, would have of course remained unmentioned.
Well if they occupy it already, it makes no sense to bomb it.