I like a kill switch, but when design it is worth giving thought to failure modes. Work out what's the most likely mode of failure while you're driving. In most cases that'll be: a wire breaking, a connection coming loose, a switch ceasing to close, or a relay ceasing to pull-in. If any of these things happens you'd prefer that the ignition would continue to operate, rather than add to the risk of breaking-down suddenly in the fast lane, or while pulling out of a difficult junction, or when overtaking. And it's also worth choosing carefully where the pull-in power is sourced from for any relay-operated systems: ideally it would come from a fuse separate from the ignition supply, so that if the relay side of the system shorts to ground it doesn't take out the ignition when it blows its fuse.
Scrotes are surely aware of the most common variant of kill switch (short coil- to ground) and, once under the bonnet, they'll make short work of it. All they'll need to start most kill-switch-disabled Minis is a piece of wire with a croc-clip, wire snips, and a piece of metal such as a screwdriver. I do have a kill switch on mine, but it's only going to stop the most casual of attacks. Whereas a battery isolator, being in a locked boot (on saloons) seems much more of an obstacle and has the added benefit of reducing risk of an electrical fire.
My main security is a padlock which fastens gearstick to handbrake. It's a visible deterrent and a it's bit unusual so a casual passing thief probably won't have the necessary tool in their pocket. Even if they are carrying strong cable snips (to cut the brake cables) they'll still struggle to have a televisual car-chase, stuck in 2nd.
