Don't know about petrol Series, but I immobilised my SIII 109 2.25 diesel by removing the return spring on the decompression system. It stopped normally, but to start it again you had to open the bonnet, reach in and manually move the lever back to the run position.

I like secret swiches but mines a diesel. I have been making up a lock for the engine stop cable but my fear is it will be lifted and towed. That's why I'm going down the tracker route. Years ago a freind had a Daimler Century Conquest, 3 left on the road, pre-selector box which few would know how to drive, not worth a lot, and still it went without trace.