This sort of relates to WW1, but really the POD is before 1900.
Anyways, around the 1870s Britain was looking for an ally on the Continent for whatever reason. In the end, their shopping turned up with France.
What if they instead went towards the fledging German Empire and Hapsburg Austria (post Napoleon Austria and later Austria-Hungary didn't seem to have a real theme other than everyone nominal loyalty to their sovereign)?
This certainly butterflies away the assassination of the archduke, but the Black hand or a Pan-Slavic terrorist organization might still end up using Serbia as a refuge and end up being sheltered there. The spark or war might be one too many blown up buildings or something else.
Or perhaps there would be no Pan Slavic terrorists (doubtful since it is hard to see how British foreign policy affects this), but France finds some way to justify retaking Lorraine.
My point being, a war between Germany and France is very likely. But then what becomes of the German strategy if it is Germany, Austria, and Britain against France, Italy (they still hate Austria), and Russia (probably still allied with France)?
The OTL plan doesn't work at all. If Britain signs on with Germany, Willhiem is going to have to pinky swear all treaties that Germany and Britain are both part of... including Belgian neutrality. The flanking move is a dud.
How about a direct assault against France? Well, France already built some fortifications. In hindsight, there are three roads (fairly far apart form each other) where a "gap" existed between forts, each allowing troop columns to go through without being molested by fort artillery. But, I don't think anyone knew of them. And even if the Germans tried this, it wouldn't work since the French could just move artillery pieces in. So forget that.
An Anglo-German cooperative invasion of France? Early in the war, Britain couldn't mobilize that many troops, so a knockout punch doesn't seem likely even with a German frontal assault and a Channel Invasion. I don't know about the Mediterranean, but unlike a channel invasion a successful landing in South of France would probably just get TTL British like OTL British in Gallipoli campaign. And even if it didn't, I doubt this would help the Germans much.
Defensive war against France? Germany made OTL plans based on the fact that they expected France and Russia to outlast them in a long war due to issues of food and manpower (and the fact that Russia won't stay behind in military organization forever). So that essentially means hoping Britain bails them out.
My guess is that if Britain allied with the Germans and Austrians, the Germans' plan for a war against France is "make stuff up as we go along since we don't have a working plan"