Re: Kill Switch
you could try installing a kill switch mounted on the transmission tunnel under the driver's seat (a la Allan Moffats 1972 Phase four, which he never raced).
Muscle car collector David Bowden said that he spent three quarters of an hour in the bowels of a ship trying to start it, until he noticed the kill switch under the driver's seat. After he flicked the switch he hit the key it started instantly. Although to install a system like this, you'd need to either place the battery in the boot and place the kill switch on the ignition wire (so when you turn the switch ,you dont disconnect the other systems from the power source, ie; radio, clock, etc).
Or you could just run the ignition wire under the carpet to the position of the kill switch, where ever you choose to put it. That way its really hard to find if a dirty no good theif tries to steal your pride and joy.
I hope this has helped you, even though the model of car is different (XA instead of XF) it should be relatively easy to adapt the principle.