Police spotting (junction watch)

The rule is the same for red lights in a car - one of the acceptable reasons to go through is that you were going too fast to safely stop when the lights changed. That's why people speed up to "catch" the lights

One of the acceptable reasons to drive through a red light?

Really?

Brun's scepticism is correct. s785 has confused his colours
http://www.legislation.gov.uk/uksi/2002/3113/regulation/36/made

the amber signal shall, when shown alone, convey the same prohibition as the red signal, except that, as respects any vehicle which is so close to the stop line that it cannot safely be stopped without proceeding beyond the stop line, it shall convey the same indication as the green signal or green arrow signal which was shown immediately before it;