It is an offence to stop in an advance stop line box under the Road Traffic Act 1988 sect 36 & TSRGD Schedule 14 part 1.

Rule 178 of The Highway Code states: “Advanced stop lines. Some signal-controlled junctions have advanced stop lines to allow cyclists to be positioned ahead of other traffic. Motorists, including motorcyclists, MUST stop at the first white line reached if the lights are amber or red and should avoid blocking the way or encroaching on the marked area at other times, e.g. if the junction ahead is blocked. If your vehicle has proceeded over the first white line at the time that the signal goes red, you should stop as soon as possible and MUST stop at the second white line. Allow cyclists, including any moving or waiting alongside you, enough time and space to move off when the green signal shows.”

Enforcement is a matter for the Police.