Just re read the SVA manual.
Application:
All vehicles except
a goods vehicles
b Passenger vehicles that comply with the requirements of item E1 of ESVA requirements and having a date of manufacture on/after 1 October 1998.
Procedure and standards:
1. Check, in addition to normal "ignition" switch, the vehicle is permanently fitted with an anti-theif device that can be activaated to prevent the vehicle to be driven or moved under iit's own power.
The manual speaks like this all the way through. Like solicitors!
So looks like I need to keep the ignition switch.
HTH