I got sick and tired of disconnecting/reconnecting the battery while I was testing the electrics so I fitted a Battery isolation switch.
For those that don't know, if you remove that red key the car is then completely dead electrically and will not start. It's official purpose is to make it more difficult to steal so that might come in useful if I ever have to leave it somewhere.
Although you can buy those keys for £1.50 off E-Bay so I guess a professional thief would carry one around with him,
But it would deter the casual thief and stop those people who think it is clever to reach in and turn on the hazard warning lights to drain the battery.