Very stupid move if true.
It is very stupid for Iran to go after British sailors because Tony Blair isn't very popular within his own party and this could only help Blair politically.
I don't think it's a stupid move. The Iranians have a history of taking hostages and coming out smelling like roses. They know that the West will negotiate in secret.
They can use them as leverage against attacks on the nuke sites or place them as shields. Not alot of great responses by the UK here, unless you concede their deaths and attack in response and that is not the way the West doe business.