Shipping problems, because the empire is 400% further than the US. Ireland and Canada were already tapped out. Relying on the empire means effectively cutting shipping tonnage down to 25% of what it was vs. importing from the US. Also that doesn't help France.
Rationing to German levels would really hurt domestic morale, which was already not so hot as of 1917. Basically it would really bring home how badly the war was going. Plus tighter rationing at home doesn't get the military the oil and other materials it needs to fight.
Britain had already used up all their collateral that the US would accept and in fact was financing the entire Entente war effort as France was basically tapped out of collateral in 1914 due to worse finances already (they were heavily leveraged in loans to Russia), while Russia was relying on the Brits already to borrow for them due to their poor finances (they were relying on French loans to industrialize). So that avenue is already used up. This is all covered in the Hew Strachan book I linked earlier and a couple others I could list if you want.
The way the OTL offensives were planned and run was entirely because of the US coming with it's fresh manpower and material. It is repeatedly cited in the historical record. The situation you're talking about was OTL 1918 after the German offensives had failed, there was serious planning for a naval suicide run, and because it was clear the war was lost given the success of the Allied 1918 offensives. That wouldn't be TTL with the US out, the Entente broke, and no clear signal that the Central Powers were beaten in the field. Besides the lack of financing to continue the war from the Entente was already coming to a head in April 1917 and it was only US entry and immediate provisions for unsecured loans that kept the war going, so by 1918 ITTL the war would likely be over or at least heavily in the CP's favor, as the key supplies for the Entente that enabled them to fight was gone.