Bike Theft Experiment - Youtube Video

Preventing crime by committing crime. Excellent. As the law stands, you can't legally physically attack a thief (particularly not with what might be considered a deadly weapon) in order to prevent a crime.

Bullshit.
I am no lawyer but I do know that reasonable force is not just ethically justified [speaking for self] but legally justified.

(1)A person may use such force as is reasonable in the circumstances in the prevention of crime, or in effecting or assisting in the lawful arrest of offenders or suspected offenders or of persons unlawfully at large.

And in any case, I was proposing a threat, which in my experience has validated the idea that

criminals are a cowardly sort

Your point about the D-Lock being attached to my bike is however valid.

@Fefelarue. You've not really answered my question. Why not do it with your own bike?
Using people's property as a prop for your video without asking for permission is rude at best.