Just my two pence worth;
You commit the offence by hitting or going over the solid white line. It is an absolute offence so you have no defence to it. People say it will be thrown out at court etc etc, but the courts will generally look at the facts, what is an offence and what is not, plus any exemptions or defences in law. The police themselves would most likely be there ones that exercise discretion and not pursue it. People have and still will be fined for it - as people have linked to. Generally only exception is where a police officer physically directs you through (this could be them getting out the car and asking you to move).
As someone who until last month played round on blue lights, "blue light bullying" is quite big on the training agenda, although some services seem to loose sight of it more than others. Blue lights are not a right to cut through traffic and do what you want (as you can still be prosecuted for careless driving etc, and the courts have already said blue light drivers are no different to the public when driving). Blue lights are merely an indication to other road users of their presence, and an invitation to give way. At a blocked junction, really emergency service drivers should leave space and turn sirens off and wait for the lights to turn green, then wait for space. Clearly this may not happen all the time (due to the circumstances of a call etc), and some may go into junction fast and bull horn everyone out the way! But that's how it's meant to happen, but there is definately too much "bullying".
Anyway.
