If a police officer in a patrol vehicle signals you by flashing their headlights, activating blue lights and/or sirens, and then indicates left by pointing to the left, it is to instruct you to stop. Pull over on the left side of the road as soon as it is safe to do so.
Traffic officers typically use amber flashing lights, usually from behind, and will point or indicate left to signal you to pull over.
In England and Wales, traffic officers operate on motorways and certain ‘A’ class roads. Failure to comply with directions given by police or traffic officers is considered an offense.