The UK’s legal right to seize either ships or goods in transit through the Strait of Gibraltar is highly debatable. It is Iran’s oil; and even if it is going to Syria as the UK claims, Syria is not under UN sanctions, the only legal international sanctions that count.
The European External Action Service, the EU body responsible for its foreign policy, was deafening in its silence on the UK action, supposedly taken to enforce EU sanctions. Even Carl Bildt, the co-chairman of the European Council on Foreign Relations, was constrained to tweet, “One refers to EU sanctions against Syria, but Iran is not a member of EU. And EU as a principle doesn’t impose its sanctions on others. That’s what the US does.”