Rule 110 of the Highway Code says: “Only flash your headlights to let other road users know that you are there.” Rule 111 adds: “Never assume that flashing headlights is a signal inviting you to proceed.”
British drivers flash their headlights to let another driver out of a junction all the same. What Car? magazine calls it one of the most common unwritten rules, which drivers are “expected to do by their fellow motorists.” In 1973, an MP asked the government to introduce an official system of flashing for give-way purposes. The minister replied: “No, Sir.”