Well, as long as they don't try to hit the one on the left, we're alright.
Seriously though, attack on nuclear power plants? I don't know about Europe, but certainly Sizewell has got a fair bit of security, particularly on the inside of the stations, ok...the outside station security is a complete joke, as Greenpeace protestors demonstrated, but to get inside you have to go through several security and radiation checkpoints. The dome itself is armoured against a nearby nuclear strike, let alone a plane being dropped in it. So what are they going to use? There are so many backups and failsafes in Sizewell that it would be very hard to set it to meltdown. Now, the last time I checked Iran didn't have a Carrier sitting in the North Sea, so they can't launch a bunker-buster air strike, and their Kilos probably wouldn't be allowed to leave the Gulf, particularly considering they'd have to refuel.
So, there'd be a lot of panic and the environmental and safety groups ('Shut down Sizewell') would have a fit if someone tried to bomb the dome, but I do doubt very much that there would be a major release of radiation, the Dome has been dropping the odd leak for years and I've not started glowing yet.
Oh, don't worry, I'm not totally dismissing the Iranian threat, I'm just curious as how they expect to carry that threat out.