Well, with a Defender you could move the seat all the way forward and run a padlock and chain from the release bar on the front of the seat and through the steering wheel. Can't sit in the seat or steer. It's not high tech but it's simple to do and anything that buys you time is worth doing.... If you use a good heavy hardened chain and a good quality anti-pick padlock it's even better. Might be worth beefing up the release bar on the seat to prevent it being hack sawed.
With electric seats you can wire in a hidden cut-out switch to lock the seat after you have moved it to the fully forward and highest position.