castiel is a director of a bus operator group, and the legislation for drivers (and of course the operators/directors) falls under the duties under the Road Traffic Act 1988 and the Transport and Works Act 1992 as well as the health and safety at work act. in summary this means that drivers of road vehicles must not be under the influence of alcohol while driving, attempting to drive or when they are in charge of a vehicle. this becomes an immediately prosecutable legal issue. although unlikely, this could mean jail time for castiel, more likely is the suspension of the operators licence for the bus company by the traffic commisioner which means no buses = big problem.
it is the 'attempting to drive' a vehicle part that is the problem here, and for a bus driver, coming into work drunk would be considered an attempt to drive the vehicle.
thus for the public transport sector, it is often more stringent when it comes to the consumption of alcohol or drugs. i imagine castiel’s company policy will not even allow employees on site who may be under the influence of drugs or alcohol for the above reasons.
for most workplaces, it is the appliance of company policy that will to fulfil / discharge their duties of the health and safety at work act. this may simply state employees may not operate machines etc under the influence but not solely prohibit coming on site.