You can use force to protect yourself, an other or your property, but as said before the force must be reasonable and you must be able to justify this. You dont need to be attacked before you use force as common law comes into it..but you must show that you believed that unlawful violence was imminent and your force was such to repel/prevent such attack.

Read R v Heggarty or R v Beckford for more information with regards to case law.

Section 3 Criminal Law Act 1967


If your talking about just doing someone with some tools which you have laying around in your shed just because they are there..then I think you will be hard pushed to justify that.. even under instant arming

The bottom line is from reading your above it... Just because he is stealing your things you dont have the right to stab them.. sorry but you would be gripping the rail for that one