The Highway Code applies to England, Scotland and Wales and is essential reading for everyone.
Flashing headlights. Only flash your headlights to let other road users know that you are there. Do not flash your headlights to convey any other message or intimidate other road users.
Comments : Its a common practice to flash at a car to let them in or allow them to carry out a particular task. If you do use a flash to convey a message and that motorist causes an accident as a result, there is a good argument that it was caused by your failure to follow Rule 10 of the Highway Code. That you are negligent. Thus be careful and observe Rule 10 and do not use it to convey a message.