As I understand it, it is legal to drive a vehicle to a pre-booked MOT when it does not have a current MOT, but not legal to do so when it is SORNed. I stand ready to be corrected, as happens often enough.
When you can drive your vehicle
You can only drive a vehicle with a SORN on a public road to go to or from a pre-booked MOT or other testing appointment.Taken from
When you need to make a SORN
How and when to make a SORN (Statutory Off Road Notification) to stop taxing and insuring your vehicle, including when the SORN is valid, penalties and rules for motor traders.
www.gov.uk