If the British Expeditionary Force is forced back to off of the continent, Germany has beaten Britain. Belgium would have signed a separate peace (In OTL they sent out feelers to open peace negotiations in July 1918, but quickly stopped after the Marne counterattack; their surrender is practically garunteed if Britain is forced off the continent). France goes soon after.
The British leadership (civilian and military) knew that the war was effectively over if France was knocked out. With the Germans in control of the Channel ports, the Dover Barrage can’t be maintained and traffic in the channel grinds to a halt, resulting in shortages in Britain. Meanwhile goods can enter Germany through now neutral France as well as overland from Spain and Portugal. The British blockade becomes a farce. They have no leg to stand on in any negotiations. If they want the Germans to concede at all on Belgium, they will have to do some bargaining.
The idea that is often thrown around that all British colonies would be untouchable sacred land in peace negotiations is stupid, colonies were exchanged all the time in peace deals historically. They weren’t considered a core part of a nation’s territory. Britain would likely be forced to give up at least part of their colonies in Africa, likely in exchange for Togoland, which the German foreign ministry was apparently willing to part with.
Wouldn't France end up in some kind of civil war post-conflict? Since the Germans have Alsace-Lorraine, they will probably ask for a good amount of reparations, but nowhere near as much as was asked by Germany. The majority of reparations would be in the form of French African colonies to fulfill the French part of the MittelAfrika dream.
I was thinking that Britan would sue for peace, as even if America got involved, I believe they'd have no way to hurt the Germans, as entry through France is how they fought. So the Germans can just keep up the U-boats, and while they were losing strength, they still can hurt the British economy, and many in Britan will want a peace deal with the Germans.