Not true. During times of war the custom was ships could well be searched and there were various rules about it. The Royal navy would not hsistate to impose it will on the seas in later wars. All teh ocnventions signed by the major powers, had various rules about blockade and stopping neutral vessels. There was no international treaty signed by Britian that made such an action an act of war. And customary pratcice it was generally allowed.You're dodging the fact that stopping a foreign vessel on the high seas and boarding without permission is an act of war.