The rules of engagement, as confirmed by Commodore Lambert in an interview with the BBC, did not allow a response to the incident. Paraphrasing, he said the the crews acted in a professional manner and followed the ROE properly. The HMS Cornwall had a gunship up during the entire boarding operation and Iranian capture of the Brits. The helo also identified the Iranian patrol boats as they were leaving port and moving at a heading directly toward to boarded freighter. The on-board inspection was complete and the Brits were heading back to the Cornwall when they were captured.
As I understand it the US has different ROE's on the high seas in response to such an incident. I believe we are allowed to respond, even in peacetime, to an eminent threat to protect our assets and personnel. I am not entirely sure of that, but perhaps some of our JAG friends here at FR can clearify that point.
ROE's are established by national command structures, which can vary by country, even when multiple countries are part of a coalition, as is the interdiction operation in the Persian Gulf.
Whether or not the Brits could have stopped the taking of the Sailors/Marines by the Iranians is an open question.
True and we'll possibly never know. The only way to test the answer is for them to have acted in the first place. I wonder if the Brits will now change their ROE?