I don't see the current mentality being able to respond to this seizure with anything other than talk, or maybe a capitulation with a prisoner exchange.
Most likely, the Iranians will release them once they have what they want, whatever that may be. The fact that they did this in the first place speaks volumes of their 'respect' towards the U.K.
It is a delicate situation. But the Iranians have also seized U.S. personnel in the past. What does that tell you of their 'respect' for the U.S.? The Iranians are opportunists full-stop. They have had a lot of success in embarassing Western states in the past (e.g: U.S. Embassy incident). But they'll be smart enough not to push their luck in the current climate. Last time they seized some U.K personnel (and briefly U.S. on another occasion)these were released quickly in this regard. The UK government will never release prisoners in exchange for these men (just see the UK and US condemnation of the Italian capitulation in Afghanistan recently).