Here's what the highway code has to say on the matter-
"194: Emergency vehicles. You should look and listen for ambulances, fire engines, police or other emergency vehicles using flashing blue, red or green lights, headlights or sirens. When one approaches do not panic. Consider the route of the emergency vehicle and take appropriate action to let it pass. If necessary, pull to the side of the road and stop, but do not endanger other road users. "
So basically it's the application of common sense- I would say that a fire engine driver or ambulance crew would probably expect you to move to the left due to the sheer size of their vehicles, but a police car is just as likely to go to the left of you as the right.