In the UK everything you do at work is covered by the Health & Safety at Work Act 1974. Therefore self-positioning would come under those rules. A key part of that legislation is that you are responsible to yourself and your colleagues for safety at work. Therefore whether it’s you on your own, or you driving colleagues, a decision to drive when fatigued puts you in the wrong, not your employer. Knowing this, you can then decide whether management pressure (or a queue at the HR door) makes you want to risk death or serious injury, and the further implications with your colleagues.