While this may seem reasonable in some light think about it for a second, a private post to a website jokingly wanting to blow up a building because of frustration has just cost a man his career. Paul Chambers tweeted:
"Crap! Robin Hood Airport is closed. You’ve got a week… otherwise I’m blowing the airport sky high!"and (because he wanted to be an accountant for which you require a clean slate) has lost his job, £1,000 and a lot of time. All because of one stupid message. Now if he'd said:
"I'm going to blow up Robin Hood airport"and then phoned the police with this message he would have made a hoax bomb threat and be liable for conviction because shutting down an airport for a bomb threat is no laughing matter. Whining about being stuck in the UK for a little longer and wanting to take out your frustration is another, especially when the airport clearly could see that it was a joke, in fact so could the police and the CPS. Which is why he wasn't convicted for making a bomb threat, but for mis-use of telephonic communication which is based upon a law designed to stop cold callers and stalkers.
Anyway Jack of kent has a much better summary and more detailed (and correct) legal information, I highly recommend you read what he has to say because you never know when an innocent message like:
"I want to put thermite (rust, aluminium ground and mixed then set alight with magnesium) through the engine block of the car parked near where I live who's alarm goes off every 2 hours"spoken in frustration and anger before being sent out into the world may get you convicted for making menacing noises.