Pilots' discretion is just that.
No-one can legally force a pilot to fly against his judgement, if he/she considers the flight cannot be safely made, for any reason (as per the requirements of the Air Navigation Order).
I've turned down a few "priority" jobs in my time, military, SAR, Police, Casevac and corporate/VIP (never without good reason, I might add) and occasionally made myself very unpopular by doing so. But from hard experience, if it all goes badly wrong, there's only ever one person carrying the can with regard to the ANO, the law and civil liability. The others involved always take very rapid backward paces to avoid any implication of responsibility.