The British navy captured an Iranian oil tanker said to be taking oil to Syria.

This was clearly an act of war, and justifies military action Iran's part.

The capture took place in disputed waters near Gibraltar, and Spain has objected to it.

I can't see what relevance EU sanctions have. If the are sanctions, they prohibit certain dealings between EU entities and Syria. I don't see how an Iranian ship would come under EU sanctions, unless it is owned by a European company.

Only a blockade would purport go that far, and a blockade, if there were one, would be an act of war in itself. (Remember when Egypt blockaded Elat? Israel responded to that, legitimately, by capturing the Sinai.)

I don't think the EU has announced a blockade of Syria. Indeed, it is unable to do such a thing; the whole field of war is outside the EU's scope.

So there is some bullshit here. I hope someone sorts out the facts.