My idea was to leave the rest of the electrical system alone and only isolate the starter motor. If you isolate the battery it would cause lots of problems. 2006 RRS SC Bonatti Grey (the 55th car/van I have owned)
2005 GSX 1400 K5

You could go all James Bond with this modification and have a pop out panel with a hidden switch.

Thanks for all the thoughts and feedback. No sign of car - and not expecting any. Will contemplate various cut off options, but suspect the best way forward will be along 'physical' protection line.....a dirty great retractable bollard in front of the car. Simple, not too expensive and not relying on not-so-clever electronics. Might even go for the old yellow steering clamp too....belt and braces, locking the stable door after the first horse has bolted, but hoping it will stop others going astray!

Yeah drop posts are good, I've got one.

Has there been any indication on how they did it?

If it was a simple lift onto a flatbed and sort the electronics later then no electrical solution will prevent this.

Or are they scanning the keys etc. 2016 RRS 4.4 TDV8 Autobiography Dynamic in Montalcino Red and Stealth Kit

Driven off - I saw it go after I heard them start it up. It has been suggested that the locking signal was 'blocked' when I locked it earlier in the evening. So, worth double checking that car actually locks when you instruct it to do so! Insurance company say there are some very professional and technolgy enabled eastern europeans knocking around, stealing to order and shipping out very quickly. Takes them less than a few minutes to get in and drive off..........and they know where to look in terms of where the typical tracking device is located. Hope this helps others avoid what has happended to me.....

Not everyone's cup of tea but I interrupted and re-routed the fuse lead (back of fusebox) for the fuel pump in a Subaru Impreza Turbo, to a hidden switch. No way that baby going to start unless you knew where the switch was...