The offence is crossing the stop line on a red light, so if you are already past the stop line on green then no offence as far as I know, however driving headlong into traffic that has now got a green light going the other way could possibly land you in trouble.
Indeed, and the offence is committed if
any part of the car crosses the line. It can be difficult, if not impossible, for a driver to see whether the rear bumper has cleared the line.
