I would suggest that we are looking at this from the wrong direction.
Whilst there is not anything specific as to whose decision it is as to whether 9 hours or 11 hours is appropriate, there are specific responsibilities under the Health and Safety at Work Act, which requires every employee to consider the safety of themselves or others. Surely being instructed to commence work when not having had sufficient (worthwhile) rest would not be considered lawful?
There is also the HSE booklet, Driving at Work, which covers this, and many other, considerations. Whilst it is only guidance, it is certainly ‘fuel’ for any argument.
As regards LAW on the subject per se. Whilst there is a ‘stated case’ on the subject of driving when unfit through tiredness, it is some years old and without access to PNLD, I can’t readily find it. (Note to self - blag my mate’s access code from him. 
)
However, what I can find HERE, on the CPS website is a reference to it as in…
There are decided cases that provide some guidance as to the driving that courts will regard as dangerous and the following examples are typical of what we are likely to regard as dangerous driving:
· racing or competitive driving;
· speed, which is highly inappropriate for the prevailing road or traffic conditions;
· aggressive driving, such as sudden lane changes, cutting into a line of vehicles or driving much too close to the vehicle in front;
· disregard of traffic lights and other road signs, which, on an objective analysis, would appear to be deliberate;
· disregard of warnings from fellow passengers;
· overtaking which could not have been carried out safely;
· driving a vehicle with a load which presents a danger to other road users;
· where the driver is suffering from impaired ability such as having an arm or leg in plaster, or impaired eyesight;
· driving when too tired to stay awake;
· driving a vehicle knowing it has a dangerous defect;
· using a hand-held mobile phone or other hand-held electronic equipment when the driver was avoidably and dangerously distracted by that use;
· reading a newspaper/map;
· talking to and looking at a passenger where the driver was avoidably and dangerously distracted by that;
· selecting and lighting a cigarette or similar in circumstances when the driver was avoidably and dangerously distracted by that.It is not necessary to consider what the driver thought about the possible consequences of his actions: simply whether or not a competent and careful driver would have observed, appreciated and guarded against obvious and material dangers.
Add to that the new offence of Corporate Manslaughter, and it is then a short hop and a skip to argue that if an employer ‘procured’ an employee to work whilst unfit in such circumstances, and it resulted in a death, then perhaps they need to develop a taste for porridge. 
