You will be good enough to provide the text in the appropriate treaty that backs up the claim that they must take them to the country of their registration (flag state). But assuming you can't and lets take it is correct for the time being, why don't you toddle off to marine traffic or vessel finder and have a look at the flags of the vessels in the English Channel. You will find Netherlands, Mexico, Spain, China, Sweden, Liberia, Panama etc. In fact Panama has the largest ships register in the world. Do you think those who wrote the treaty were not aware that ships travel all over the world?
So I pick someone up 5 miles from the French coast and say, sorry buddy I have to take you to China.