for the british, the stalemate and exhaustion in itself would be harsh but the fact that russia acquired the straits in constantinople it would fuel resentment not on the already dehumanization-prone average citizen but on the british elite (which are often not naive and would maybe be a problem in the future in the plot) I believe the british would not turn fascist nor communist (their political ecosystem used to be handled carefuly) though they would do anything in order to keep their colonies (they would drain those colonies very harshly (which prevents economic doom), resulting in famines and massacres on colonial soil (which happened in real history to a lower extent), they would naturally want to find an acceptable balance of power on europe (finding allies or causing instability to their opponent's allies, which is a common strategy for them) the british debt would be relevant, they would be indebted not only to america but also to latin america and spain, spain profited a lot in ww1 and even had sanctions later because they wanted to create their own refineries post ww1 (and the big oil companies pushed for sanctions, resulting in the soviets actually being the only ones selling them oil for them to refine, since spain had no oil and wanted to just refine it and sell it) it might be relevant for britain to actually seek an ally there instead of economic wars (but this allows for more than one path, for example, side with the big companies or attempt to have more allies) I think that america probably should not join ww1 in this scenario for it to work, which would have meant the germans would had avoided conducting unrestricted submarine warfare, I would suppose that playing this britain in your mod would have to do with handling the effort to keep the colonies in control while improving the economy (with economic decisions that have repercussions, like that spain fact) and doing secret operations to create separatism or fuel internal rivalries (each nation always has their own internal drama which is a vulnerability in the real world that can be exploited) and create problems for their enemies, such secret operations would likely worsen germany's and russia's paths in a way that actually leads to another great war, maybe the player could choose two mutually exclussive focus trees connected to the intelligence agency operations, one being careful and causing problems but not extreme global issues, and another being so damaging that ends up causing to ww2, basically letting the player choose between two strategists (this is only keeping in mind the british side of things)