Rule 71

At traffic light junctions and at cycle-only crossings with traffic lights, you MUST NOT cross the stop line when the traffic lights are red.

Some junctions have an advanced stop line to enable you to position yourself ahead of other traffic and wait (see Rule 178). When the traffic lights are red, you may cross the first stop line, but you MUST NOT cross the final stop line.

Laws RTA 1988 sect 36 & TSRGD Schedule 14 part 1