What little old insignificant Britain being asked to stop the tanker near Gib and look after this tanker in the Straits of Hormuz. Who would have thunk that especially after all the talk Britain down rhetoric from the Remainers that the UK is like a banana republic just above Haiti in the ranks.
AIUI the tanker is the BP owned "British Heritage", which is registered in the Isle Of Man so entitled to protection from the Royal Navy. That's one of the primary reasons the RN was formed - to protect the British merchant fleet. If the Yanks got intelligence that some Iranian up to no good was about to happen, and there was a RN presence in the area, then I'd bloody well hope they pass that info on to HMG and appropriate action is taken. The yanks don't have to tell us where the info came from, spy satellites, drones, ELINT etc, but we should be thankful they shared it with us.
If there had been no RN presence then it's up to the rules of engagement of whatever assets were in the area, and their chain of command, as to what happens next. Often that is nothing, as was the case with the two tankers a few weeks ago. The US obviously had 'eyes-on' but on that occasion chose not to respond with lethal force. If the tankers had been US flagged things would have been different.
If the merchant ship is sailing under a different flag, then it's up to that country to sort their mess out. If the ship had been Panamanian for instance, I doubt HMS Montrose would have intervened so swiftly - and it's anyones guess if the USN would have reacted.