It is an offence. Best to report him to the police for an offence under Section 3 of the Road Traffic Act 1988 - driving without reasonable consideration.
The CPS guidance on this states:"This offence is appropriate when the driving amounts to a clear act of incompetence, selfishness, impatience or aggressiveness. There must, however, also be some inconvenience to other road users, for example, forcing other drivers to move over and/or brake as a consequence. The following examples are typical of what we are likely to regard as inconsiderate driving:
- flashing of lights to force other drivers in front to give way;
- misuse of any lane to avoid queuing or gain some other advantage over other drivers;
- unnecessarily remaining in an overtaking lane;
- unnecessarily slow driving or braking without good cause;
- driving with un-dipped headlights which dazzle oncoming drivers;
- driving through a puddle causing pedestrians to be splashed;
- driving a bus in such a way as to alarm passengers. "
Hope this helps