The last few generations of passenger cabs have this issue and I don't know why ASLEF accept them.
Although UK trains are de facto single driver operation, there are also so many needs for a second seat in the cab. These vary from traction training and route learning to assessments, pass outs, pilot working, line examination etc etc. Yet ASLEF seem to sign off cab after cab as fit for use whn the majority of task mentioned cannot really be achieved satisfactorily.
You don't ever see a rather pisspoor "aecondman's" seat in an aeroplane cockpit do you ?
Aircraft, unlike trains, have a constant requirement for 2 people "up-front" at all times, and the seat that is in overall command will alternate on just about every leg.
Despite that long list of tasks for the second seat on a train, I suspect that the average number of people in a cab rarely exceeds 1.05 and compromising the cab for that 5% of occasions when it's in use does not outweight the benefits of an uncompromised main driver's seat.