"So that's clear then if they are creeping they have not applied the handbrake and are breaking the law, regardless of the temperature of their discs"
Not 'quite'. The law is specifically about lights, and not using them in a way to dazzle. The 'MUST NOT' (and therefore the legal requirement) is not to dazzle or cause discomfort, specifically in this case, "Prohibited Use, 11. Any other lamp: Used so as to cause undue dazzle or discomfort to other persons using the road."
The applying the handbrake is 'should', and is best practice, so as to avoid sitting on the brake pedal, causing that dazzling. So not applying the handbrake, but also not keeping the foot on the brake pedal, is not illegal.
It's fuzzy, as with a lot of road traffic laws. Of course creeping past the stop line is an offence in its own right (and personally when driving and I'm in a queue I do the whole, car in neutral, apply tyhe handbrake, thing).