Iran broke some international law?

On what basis did the UK seize Iran's tanker. On the basis of an act of war, seizing an oil tanker is an act of war. So, the UK initiated a war with Iran, I am not sure that was a good idea, especially being in the middle of brexit and all.

And this:

Israeli minister boasts his country has been 'killing Iranians'

In this case, Iran has already declared war.  War declarations are unilateral, the UK did it to Iran, the Iranians did it to Israel. Radical Sunnis did it to Shia, and we got declared war in Kashmir. Turkey declared war on everyone.