Under the UK Highway Code Rule 110 , you should only flash your headlights to let other road users know you are there.
: Vehicle direction indicators must flash at a constant rate of 60 to 120 times per minute. uk flashers
: Only authorized emergency vehicles (police, fire, ambulance) are permitted to use blue flashing beacons. These vehicles may also use alternating headlamp flashers for better visibility. Under the UK Highway Code Rule 110 ,
: Flashing headlights to warn others of speed cameras or police checkpoints can lead to fines of up to £1,000 for obstructing police duties. uk flashers
In the context of angling, a "flasher" is a type of attractor used to lure fish.