A bit of context:
Yesterday I was near oxford street, near a T junction that has traffic lights. A few policemen in motorbikes, using whistles (not sirens) stopped in coming traffic and allowed two range rovers through. Even though the traffic lights were red.
Therefore, what circumstances are police able to use there powers to stop traffic for a convoy?
A police officer in uniform is entitled to issue directions to road users at all times. Those directions override signage and laws relating to things like traffic lights.
There are probably police rules about when they are supposed to do so (and breaking those rules would render them liable to disciplinary action), but as far as the law is concerned "whenever they feel like it".
