Crossing the stop line after red is an offence. You should bear in mind that amber also means "stop" and that is displayed for around three seconds. The police have chosen to act against you. That's all there is to it. It is entirely a matter for them to decide whether to take action or not.
It is unlikely you will be prosecuted. Although there are no published guidelines, 0.9 secs could possibly see a course offered. Failing that, a fixed penalty of £100 and 3 points.
