Under the UK Highway Code Rule 110 , you should only flash your headlights to let other road users know you are there.
: 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
: 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 ,
: Vehicle direction indicators must flash at a constant rate of 60 to 120 times per minute. These vehicles may also use alternating headlamp flashers
: These should only be used when a vehicle is stationary and causing a temporary obstruction, or to warn following drivers on motorways of an obstruction ahead. Fishing Equipment
In the context of angling, a "flasher" is a type of attractor used to lure fish.