fuel/battery cut off is a good idea but it doesnt stop someone smashing the window, taking it out of gear (come on...no ones 306 handbrake works) and towing it onto the back of a lorry.
Best thing is something that makes a lot of noise or leaves their dna behind. even cheap alarms are good providing youve installed it well. Ok no one pays any attention to car alarms going off outside but if your trying to pull back a door and the alarm goes off, your not going to hang about are you!

simple. Just put a switch in wire BB2 going into the fuel relay