Minis and many other cars up to around 1970 just had a key switch and no steering lock unless the law changed after that date to require a mechanical lock I think that would suffice.
My guess would be that the law was intended to stop cars being started accidentally, by kids playing in them and the like.
How does it stop kids starting the car by accident? if they have the keys then the lock comes off if you try and start the engine and the fact that the steering wheel doesn't turn has no bearing on the engine starting at all?
I'm just going to fit the battery cut-off and hope for the best.......