Just flicking through the IVA manual and the only reference I can find to an 'ignition switch' is...

"Check, in addition to the normal “ignition” switch, that the vehicle is permanently fitted with an anti-theft device or an electronic immobiliser that can be activated to prevent the vehicle being driven or moved under its own power... "

So what constitutes a 'normal' ignition switch? Must it be a key activated switch? Or could I simply have a suitable electronic immobiliser system, and a toggle 'ignition-on' switch?

Yes I see what you mean, the "normal" ignition switch is no more. I have assumed "normal" as is a key activated switch in my build. Don't think you can just fit a toggle switch and immobiliser as I read somewhere you need two forms of security (could be wrong).

I believe that it is interpreted to mean you need a key plus either an immobiliser or steering lock.

This has come up before. Hidden away in the "glossary of terms" at the end, it says:

"Ignition Switch
A key operated switch normally used to start the engine. "

So that's the official answer.

BUT

I passed with an immobiliser and just a toggle switch to turn on the ignition, no steering lock, no key. The only thing the inspector said about it was that it was "cool"
So I guess it just depends who you get. In reality, many cars nowadays do not have key-operated ignitions so I suspect the definition is a bit of an anachronism. If in doubt, ring the test centre and ask.