Okay there are two possibilities
One is that the UK is neutral because Germany doesn't give them a causus belli like Belgium, the other is that Britain is too distracted with something else to declare war even with the Invasion of Belgium
In the first case the French are obligingly sticking their dicks into a meatgrinder and turning the crank in Alsace-Lorraine, the Germans don't need 7 Armies in the West and can spare one to send to Prussia and maul the Russians worse there and another to dig Austria-Hungary out of their bad situation in Galicia. The result is that Russia is forced to make the major withdrawals they did in 1915 a year earlier, Austria has the reserves to finish off Serbia over the winter and France has battered itself to exhaustion for little gains. If the US is involved, then it has the troops to break the Confederate offensive across the Potomac, plus slightly speed up the other offenses, and the Navy can sail up and down the Confederate coast with impunity. Without A-H getting mauled over Fall and Winter Italy probably decides to wait longer to join the war
In the Second, the East would goes as OTL, but without the BEF the French Fifth Army is mauled if not destroyed and incapable of offensive operations at the Marne, Sixth Army is stretched to cover the positions of the BEF as well and cannot take the offensive. Logistics save Paris for now, but the Germans do not need to retreat, without the BEF the Germans win the Race to the Sea, control the coalfields at Bethune and Belgium and the coast up to the mouth of the Somme is lost. France lacks coal during the winter and armaments production is much lower than OTL. Italy again probably decides to stay neutral for now, and US goes as option one
In both cases the British not joining immediately has very bad effects on the Entente. In case one Russia has taken a very heavy blow, Serbia is off the board and Austria-Hungary is in a much, much better position, in case two French war production has cratered, instituting the Dover Barrage would be impossible, France has taken worse casualties and Paris in under threat. In either case the Confederacy has taken a rather severe beating and Italy is much more reluctant to join. Japan is also neutral and German and American raiders will ravage French holdings and commerce in the Pacific and Atlantic without the RN and IJN