(2-part post):
You're looking for a specific law which simply doesn't exist.
Many motoring (and, as here, 'motoring related') matters are not specifically defined as offences. For example there is no law which states that it's illegal to drive on the 'wrong' side of the road. If the police see you doing it, they (and the Crown Prosecution Service) have to decide if you've contravened a more general law, such as the one which prohibits 'driving without due care and attention' or the legislation which makes 'dangerous driving' an offence. (The milkman who drives his float, at 6am, down the 'wrong' side of the road is almost certainly acting completely legally, since it's unlikely that he's contravened either of those general pieces of legislation).
Similarly, you won't find any specific law stating that it's illegal to flash your lights as a warning to other drivers about a speed trap. If the police see you doing so, they (together with the CPS) will look to either statute law or common law for a more general offence.