Your implying UK does NOT have the ability to respond. Perhaps you are correct, but if this drags on like the hostage crisis under carter, what kind of message does that send?
We will probably not intercede on Britian's behalf unless we are asked, but this will only embolden Iran more if nothing is done.
The message is that if you can't stand the heat, stay out of the kitchen. Or, don't take a knife to a gun fight; or, don't play high-stakes poker if you don't have the stakes.
GB choose socialism over internationalism a long time ago. They simply do not have the military capability to respond to any threat anywhere, which is why it was foolish to put their service people at risk.
The only way out for GB is pay whatever ransom Iran will eventually demand. Then, they should withdraw their troops in a similar fashion as Spain. At present, there are only two countries in the entire world that can project force beyond their borders: Israel & the USA.
When we eventually take on Iran, NATO/EU might help out in a supporting role, but it will be our responsibility.