In a Germany-doesn't-try-USW TL, there are (get this) actually benefits to Britain.OTL, Britain may have been at the end of their tether, but who's to say that'll be the case without months of U-boats wreaking havoc.
They could still have been wreaking quite a bit.
Even before USW, total tonnage sunk had roughly tripled, from 123,100 in Dec 1915 to 355,100 in Dec 1916. In Jan 1917, the final month before USW kicked in, it rose again to 368,500. Had that been maintained through 1917, total tonnage sunk would have been around 4.4 million, compared with approx 6.3 million OTL This is indeed a substantial drop, but still about equal to 1914, 1915 and 1916 losses combined.
Also, it appears from President Wilson's actions (or inactions) about sinkings of armed merchantmen in 1916, that a declaration of USW against armed ships only would have passed muster with him. Since virtually all Allied merchantmen were armed or in process of being by 1917, this would have been the same as full USW so far as they were concerned. Sinkings of neutral ships would still have been lower than OTL, but OTOH the Allies might have lost the use of many neutral ships which might have preferred to take refuge in US ports. OTL, of course, they didn't have this option. Indeed US belligerancy meant the Entente was "the only game in town" as far as trade was concerned. There just weren't enough neutrals left to keep a shipping line in business. It was a choice between accepting the risks and trading with the Allies, or going broke.
There would also be indirect losses. If purchases from the US dried up, fewer US ships would be calling at Allied ports, while Britain would have had to buy from places willing to be paid, effectively, in IOUs. Except for Canada, virtually all of these would have been a lot further away than the US. Any given ship could make less than half as many voyages to and from, say, South Africa than to and from the US. This would probably more than offset the reduction in losses to u-boats.
Paradoxically, there would be some benefit to Britain if and when her allies collapsed, since the large part of British shipping currently engaged in supplying the latter could then be used to supply Britain herself. But in such circumstances there would seem little point in continuing the war, particularly if the RN itself was in danger of being crippled by a dwindling oil supply - probably a greater danger than food shortage.