It seems logical that since the German came close to defeating Britain in 1917, with better basing that they could win in an ATL where they controlled French ports. I am not so sure though
It seems like the British would just abandon the Channel for shipping and soon be forced soon just like OTL to put merchants into convoys (as soon as the losses hurt they will do it OTL or ATL). But since convoys work (especially in 1914-1918, where there isn't much radio signalling and little air recon to defeat convoys), that submarine warfare couldn't force Britain to the table, especially if the Germans insist on ridiculous terms.
Of course conversely with French Atlantic ports under their control, the Germans could resume limited trade (or just trade with neutral Spain) so the British Blockade wouldn't be effective.
Seems like such a war could go on for years.
Admiral Bacon wrote that the railways couldn't handle the amount of freight that was shipped directly into London so if through-Channel shipping (100-120 ships per day 1915-16, 80-100 in 1917) was stopped 'at least one third' of London would have to be evacuated. Hyperbole perhaps, but I suspect a good measure of truth.
Convoys are an interesting thing, awesome for defending against Uboats but from mid June 1916 Germany had a flotilla and a half of destroyers at Ostend and late October 1916 it was 2 1/2, so through-Channel convoys would need heavy escort which would be demanding on RN destroyers and light cruisers needed by the Grand Fleet. Indeed the radius of action of the GF was restricted in October 1916 due to a lack of destroyers.
It's the lack of use of surface forces that are the real failure of the Germans in WW1.