One of the responses available to Britain under the International LAw principle of resiprocity, wouyld be to capture its own set of Iranian Hostages. That is why Iamanutjob cancelled his trip to the UN when this incident happened. He knew he would become a guest of Her Majesty's Government.
There is a whole UN Iranian set of personnel who are involved in subterfuge against the West, who live and work in New York. Britain could easily nab 15 of them while US Security looked the other way.
Then Britain could do the same. That is an old Islamofascist Game from Persia and Arabia. Its been going on for thousands of years. Britain needs to play it, but the Liberal Socialists of the Blair government will never authorize it.
THe Queen of England might authorize it though. That would be interesting.