if its a company vehicle registered to the company then they are responsible for the vehicles tax,mot,insurance,but i believe the driver can be held responsible for certain things whilst driving it,ie lights etc road worthiness etc.but i would check it out,if you have doubts then refuse to drive it.
If it isn't taxed,then it shouldn't be on the road,so while the firm may be responsible for the taxing of vehicle, the employee was the one who took it on the road,so I guess the person in charge of vehicle,the driver.will be held to blame
your employer is responsable for taxing the vehicle BUT you as a driver you are always responsable for the vehicle and it`s load in any way when you take it out on the public hiway