Its entirely up to you... do you think that your car might have a higher chance than normal of getting wrote off etc...
I wouldn't really bother for a car that costs £11-12,000 as its expensive and the chances of that actually happening are very very low...
For my Focus
I might consider it as I'm expecting that to be a £27,000 car which will be higher on theft list... but even then I won't worry about it.