A pre-Worboys Committee prohibited traffic sign. 
Difference between 'Red Disk' and 'Red Ring':

Red Disk: Prohibited or restriction against all vehicles: "No left turn" and "One Vehicle only on bridge Maximum Weight 15 tons Speed limit 5 mph".
Red Ring: Mandatory or restrictions against some vehicles; "Keep Left", "Speed Limit on Bridge 5 MPH for vehicles over 5 tons gross weight", and Speed limits."
Report of the Departmental Committee on Traffic Signs - 1944.
Ministry of War Transport