Do police drivers abuse their powers?

I would particularly value Bob's opinion on this incident, which took place this morning.

I was driving towards my local town centre when, about 100 yards ahead there was a marked police Vauxhall Astra waiting to turn right out of a side road. Traffic was fairly heavy and, as I approached this junction, the Astra's blue roof lights came on. Thinking that this indicated that the officers in the car were required to attend an emergency 999 call, I slowed and allowed the police vehicle to pull out. However, as soon as they had pulled out into traffic, the blue lights went out and the Astra then just kept up with the rest of the traffic which was travelling at around 20mph.

It would seem that, in this instance, the officers had used the blue lights on their car to effectively bully their way out of this junction. I had always assumed that police drivers were supposed to set an example to the rest of us but, had it been me who had used the same sort of tactics and been observed by the police doing so, I have no doubt they would have stopped me.

What's the legal position on such a move Bob?