I got the distinct feeling that the effort was potentially dangerous, considering that the taxi driver may have booked on hours prior. I expect there are the same regs in force for taxi drivers as for HGV drivers but I wondered, what did the taxi driver do on reaching Cornwall - turn around and drive straight back? It sounds dead dodgy to me.
There is not a regulation of how many hours they can drive in a shift. There are recommendations but are not compulsary. I know of self employed taxi drivers who start as soon as they get up in the morning, and work until they are ready to go to bed.