Sad fact they are desirable to thieves but nobody would buy one if worried about theft, if you decide to buy one get as much security on it as you can and use a bit of common sense when parking it up.
It'll be fine. If you're that concerned then don't get one. However, an immobiliser/battery kill switch and crooklock will deter most casual or opportunistic thieves. It's the groups and planned ones you have to worry about. Even then, depending on your vehicle they may not bother. Look into pedal locks and a gps tracker. Both things i need to get my series. You've got to remember that if someone is determined enough, they'll take it. Regardless of the vehicle, regardless of the security. That's the case with anything in this life. Just that Defenders are pretty desirable. Get a tracker, stamp your numberplate into different parts of the body that are hidden, get the glass etched.
You will never stop it but you can deter it, make it difficult and ideally with a GPS have it tracked and go get it. They're cheap too.