The CDF say
n August, Alex Paxton was issued with a fixed penalty notice (FPN) for failing to stop behind the Advanced Stop Line (ASL) at a set of traffic lights. He did not stop behind the line (i.e. in the cycle box) because a car was illegally positioned in the box. For his own safety he positioned himself ahead of the car, past the white line. A police officer witnessed this incident and radioed a colleague, who stopped Alex further down the road and gave him the FPN. This officer had not seen the alleged offence, so could not assess the greater risk Alex would have been in had he positioned himself behind the white line, and therefore did not accept Alex's justification of his actions.
The Highway Code encourages cyclists to 'use cycle routes, advanced stop lines, cycle boxes and toucan crossings unless at the time is it unsafe to do so'. Alex's adoption of a safer position is therefore entirely justified.
IMO the correct legal place to stop would have been behind the first car in line. In real life I might well do exactly what this cyclist did (after checking for the police) but if I was caught then that was the chance I took.