One has to wonder...but the British would not have gone over to the German side and would have even pressured the Belgians to letting the French thru. The British were not going to allow the Germans to dominate the continent.
I think there is a bad case of post-WWII hindsight here. The Entente Cordiale and the realignment against Germany was still a very shaky halfhearted thing by 1914, only Belgium cemented it into stone. If not for the naval race and terrible diplomatic bungling by the Kaiser it would not have happened at all and UK would have allied withe germany sometime in 1890-1910 (there were serious mutual attempts at a German-British detente as late as 1912-13). Until a few years ago France was the hereditary enemy and Russia the feared strategic rival. If Germany goes east and France invades Belgium in the minds of the vast majority of the British public it will be Louis XIV or Napoleon bully France raping the Low Countries and making yet another attempt to dominate Europe all over again. Sure, there might be a few Germanophobe hawks in the British elite but they will meet an impassable wll of scorn in the Parliament and the public if they dare propose siding with France and Russia. Think Churchill at Munich. Britain will at the very least stay neutral throughout the war.
Heck, if the Central Powers play the diplomatic and propaganda game well, they highlight the Napoleon analogy, since the Belgians are going to resist French invasion any bit as forcefully as they did the German one, and they will loudly call for aid from anybody, most definitely including UK and Germany, and the invading French blatantly misbehave any bit as bad as the OTL Germans, there is a realistic chance that a strong anti-French reversal in the British public opinion and Parliament will bring a pro-CP government to power and a British DoW against France. The plots of some Germanophobe ministers to hatch a flimsly casus belli against Germany won't go absolutely anywhere in this political atmosphere.
So UK is either neutral or CP with a French rape of Belgium. Say 65-35%. This also means a neutral USA or a late entry for the CP if France messes too much with the USA merchant shipping to Germany. Somewhat more likely if Republicans win in 1916. French rape of Belgium paints France as the bully of Europe in America and it stays that way.
There is a realistic chance that Netherlands will enter the war on the side of Germany when France invades Belgium. It does not change the strategic equation too much unless UK sides with the CP, but it makes the Alliance look even more good among neutrals.
Germany never suffers any blockade. This alone would garantee victory, as it would the fact that French manpower will never withstand the onslaught of the vast majority of the CP Armies after Russia collapses. But there's more.
With Britain out of the Entente, Italy will never defect to that side, too. With France clearly the aggressor, there is a very strong chance that they entry the war for the CP in 1914, if not it will be in early 1915 after getting a garantee about the satisfaction of their plentiful French claims from their CP allies ("Trento and Trieste ? Bah, let's pick what's more affordable, Nice, Savoy, and Tunis, join the strongest side, cripple rival France, and look very good by being loyal to our old allies with a good casus belli. Austria-Hungary will be the job for another generation"). They are not likey going to make any big inroads on the Alps without Schliffen but they tie down a significant slice of the French Army, send several divisions to butter up the front in A-L and Belgium, and 100% of the A-H Army is free for Serbia and Russia. They also send an expeditionary corps in Albania and Montenegro, so either this spells the immediate collapse of Serbia or when Bulgary attacks the whole Serbian Army is encircled and it surrenders.
With Britain neutral or CP, it will take a little diplomatic mollifying for Germany to take the Ottomans in the Alliance with Italy already in it, but with hereditary enemy Russia on the other side and claims to pick in the Caucasus, it will most likely happen roughly on schedule. Where the Quadruple Alliance goes, so does Bulgaria to reclaim Macedonia, so Serbia goes out.
French fleet now is badly overstretched from the North Sea to the Mediterranean, losing control of both. Blockade won't hurt them as bad as the Germans, but it hurts, and the French Empire is in serious trouble, with communications with the homeland all but cut off. Germany and Italy eat it bit by bit. No colonial troops for France. Paris already has very serious manpower troubles in the long term against the Italo-German combination and a front stretching from Flanders to Nice and no British help and each foolhardy "elan" offensive will only make it worse.
This all means that Greece is a friendly neutral which smiles and nods when the Alliance speaks.
Barring exceedingly good Entente diplomatic skill or temporary show of strength, Romania stays neutral or realizes the only Romanian realistic claims in this situation are Bessarabia and Odessa, so they might well attack Russia.
Sweden has a pro-German King and Army, and ITTL the CP are getting a lot more sympathy, and they have a strong interest in expelling the Russians out of Finland and Estonia. They have the difficulty of a socialist Parliament which is pacifist on principle, but if the Russians make some aggressive move (they came very close to attacking the Swedish fleet OTL) or Germany plays the diplomatic-propaganda card of the liberation of Finland weel, they might enter the war, either in 1914 (the former casus belli) or in 1915-16 (the latter).
Japan very likely realizes sometime in 1915 that there is still plenty of booty to be picked in Manchuria and Siberia and the opportunity is just too good.
Now FR-RU vs. DE-AH-IT-OE is already a death sentence for the Entente in 2-3 years. Each additional front by Japan, Sweden, and Romania accelerates the collapse of Russia by say a season or two.
As for ideas about the kind of peace deal that France will get, see my
Reverse Versailles (the PoD is different here, so some things may seem strange, ignore references to Belgium ceding territory). Russia gets the usual Brest-Litovsk treatment, only worse. Even in the most favourable case, there is going to need at least 1-2 years to bring Russia to collapse. No peace by Christmas. The length of the war and the loss of blood and mony won't be nowhere as hard to the CPs as OTL, but substantial enough that France and Russia will get an harsh peace deal. Besides, ITTL France has been the aggressor for three times in a century (Napoleon, 1870, and 1914). Not going to find much lenience.