Dearieme said something quite interesting in the comments to this brief piece linking to a lengthy and pleasingly outraged rant at Ephems of Brian Barder. There's something understandably quite appealing about the frisson of excitement to be gained from re-imagining the past, from playing speculative games with something that, unlike the present, since it is fixed, cannot play them back. The case that Dearieme suggests as the point at which normality collapsed is quite plausible, it seems to me:
I sometimes wonder whether "normality" is the hypothetical state the country would have reached if its evolution after 1913 had not been distorted by two World Wars.
You could go further back, perhaps, but coming further forwards seems harder: the near past, more recent in our minds, its immediate causal ancestors also closer to present concerns, seems somehow more fixed than a past which is now beyond the memories of almost everyone now living. It is more implicated in the present perhaps, lacks the distance to be different, since doing so would make us too different to be us, maybe almost unrecognisable, despite the fact that, since we live now, the chances are most of us would live in most other nows, and so we must be us.
Going further back, although easier I think, in extremis seems to present a kind of reverse version of the same problem: rather than resulting in a kind of disquiet from the alterations a change would make to us, a kind of excessive openness, going too far back, to say medieval times, is too closed, has too many fixed points that need to be changed. Rather than disorientation occurring because things are similar without being the same, it occurs because things are too dissimilar to be anything like the same. Just as pondering what would have happened if the Gang of Four hadn't split the centre-left vote in 1983, or Argentina hadn't invaded the Falklands, brings the prospect of a series of selves which are both us and not-us, sets of weird, separated twins unable to reconcile their similarities with their profound differences, pondering what would have happened if Henry VIII had been able to have a child with Catherine of Aragon opens up the possibility of something like the children your parents had in another life, in another country: an insistence on a meaningful relationship despite all the evidence, other than that of the counter-factual, that strains at credulity, even comprehension.
So I like the idea of a world without the First World War: close enough to make sense, and far enough away to keep contingency at bay. Just in Britain, we could imagine a Liberal Party that didn't split under the strain of running a total war, and then institutionalise that split in the face of being squeezed both from the left and the right; Home Rule for Ireland without 1916 and its aftermath; Reform of the House of Lords; perhaps a better response to, or even the avoidance of the slump of the 1920s; no enormous national debt accumulated from funding our Allies. Globally, who knows, whether, for example, Tsarist Russia would have been able to hobble towards some kind of quasi-constitutional liberal state in the absence of the pressure of war that it suffered horribly in, and whether, indeed, that would have made for, globally, a better world. A world without the First World War might be a considerably more hospitable place.
It might not be though. Assessing how long the various multi-national empires of Central and Eastern Europe and the Near East and their resentful successor states could have staggered on, shedding and gaining territory, opening opportunities for and creating requirement of international realpolitik is, I think, likely to be rather difficult, as making accurate guesses to the consequences of the various demises of the various states also is. Equally, domestically, whilst Asquith and Lloyd George's differences, exacerbated by the War, certainly started the liberal collapse in the 'real' Britain, tensions over Ireland or the scope of domestic reform could easily have brought similar tensions to the fore. After all, famously, if you'd told people in 1913 that Britain would be fighting a war the next summer, they'd have assumed it would be in Ireland.
One of the people doing a doctorate in political theory I know is working on establishing a framework for thinking about compensation for historical injustices, in which counterfactuals play a role, as they are needed to assess the level of damages which themselves constitute the injustices. One of the limiting features of his model is that the counterfactual used to assess the damages needs, in order to attribute moral responsibility properly, to imagine a world in which neither serious wrongs nor supererogatory acts are committed, where damages are not either increased or diminished by the intervention of other actors. That, of course, seriously limits the number of counterfactual worlds, yet his model incorporates the possibility of reasonable disagreement about the content of those worlds. In a much larger set, that disagreement is likely to be all the greater.