First thing the Grace 1 has been owned by a ‘Russian’ company based in Dubai(Russian Titan Shipping Lines) web site company name etc all in English so you dont miss it, since 2018, its flying the Panama flag and may or may not have Iranian oil destined for Syria, it probably has Iranian oil based for Russia, this is not an IRANIAN ship.
Secondly the EU sanctions violated was over dual use goods from 2009 and it is not Iranian but Syrian sanctions (Article 4(2) of Regulation (EC) No 428/2009) and yes the ship was in Spanish waters and yes it was lawful for the UK to take the ship for breaking the law in this case specifically that mentions on ‘transit’ of goods across the EU.
Next this particular vessel sized by Iran is Swedish with a company name plaque on an address in Glasgow it was already parked outside the UAE fujairah oil terminus where it was meant to take on UAE oil but for reasons best known to the company decided to make a run for Saudi oil instead without waiting for HMS Montrose to arrive for escort, not in fact asking for assistance.
They know better now. The situation on Iran’s side is entirely in this instance its own fault, Iran had no jurisdiction over the waters claimed since as recognised international waters, and they themselves confirmed such during the shooting down of the US drone any action against ship or crew in law comes under an UN Admiralty court.