HR Advice. Temperature in day cab

Personally I would say stuff them, let them try to discipline, but I know everyone is not the same; I came across this on a business forum and wondered what the actual legal position would be.

HR advice needed please. Lorry driver - Night worker - Rigid HGV - Day Cab (NO night heater) - Temperature last night was -2 (but has been -6 etc) - has been told by company he is not allowed to keep engine running idle for heat in the cab - if he does he will be under disciplinary - is that legal?! I know there are rules for temperature in offices - but not sure about HGV drivers. Thanks in advance.

Time for a new job methinks, I wouldn’t be putting up with that.