The Mystery
I understand what you mean by feeling pressured (and the problem of reporting fatigue), but it would be difficult to establish a breach of duty of care by an employer if the pressure was based simply on an employee's feeling.
The position would be different if an accident occurred during the course of work and it could be proved that the employer put pressure on an employee to continue working, ignoring complaints/protests of fatigue.
bjcc
You need to differentiate between:
(a) criminal offences (not under discussion here) and civil proceedings arising from an accident,
and
(b) whether the accident occurred on the way to/from work or during the course of work.
eg Fatigued employee falls asleep at the wheel and is prosecuted for a road traffic offence:
No defence that he was tired because his employer forced him to work long hours - regardless of whether the incident occurred on the way to/from work or during the course of work.
eg Fatigued employee, tired because his employer forced him to work long hours, falls asleep at the wheel, has an accident which gives rise to a civil claim:
On his way to/from work - unlikely a claim against the employer would succeed, but there are circumstances in which it might.
During the course of work - if it could be proved the employer put pressure on him to continue working, then the employer may well be held liable - especially if the employer ignored complaints/protests of fatigue.
The extent of the employer's liability would depend upon the facts of the particular case. eg A court might decide the employee is X% liable for continuing when he knew he was fatigued and the employer Y% liable for putting pressue on him to do so.
You cite the Highway Code "as an indication that it is an individual responsibility to decide to drive, or not when tired." That is of limited relevance to a civil claim based on alleged breach of duty of care.
In a civil claim, it is only a factor which would be taken into account. It is not a complete answer, and would not absolve an employer from liability. The courts realise that employees can be put under enormous pressure to comply with employers' demands - no defence to a prosecution, but very relevant in a civil claim.
With respect, your approach is too simplistic and may mislead.