What constitutes an acceptable security device?

Ignition key switch

Steering lock

Both

Ta

Mike

mine past sva with steering lock and ing key

Yes, ignition switch and steering lock are counted as two separate devices.

i junked Sierra switchgear and had the upper column bearing casting machined and polished - ie no steering lock.

I passed with a key switch on dash (that you turn before pressing start button) and a transponder type immobiliser. just to give you an option.

I think others have passed with a battery cut off switch under the bonnet as one of the devices too.

Yup. I passed with an ignition switch and a battery immobilizer hidden away. There was discussion about whether the battery cut-off should or shouldn't be reach-able from the driving position for it to count (ie if its stuck in the engine bay you're unlikely to use it but if its too obvious its hardly a security device)...

I passed with ignition key and fuel pump immobilisor switch

Steve

mcMaster Carr in US carries high security switches that can be wired to cut ignition only or fuel only.
That way even if the burglar manages to make the engine turn, the out of sight switch will prevent from running.
check page 827 for figure...