I've spoken to a police officer about this in a kung fu class - you can hit an intruder ONLY if you believe he is going to hit you first. It's called pre-emptive strike and is very much legal luckily.

Still, the thought of someone trying to steal my dog and even break into my house for such a thing terrifies me, Oscar's a black shih tzu, so quite uncommon, though he's neutered now so that'll have lowered his monetary value...I'll have to warn my mum, she walks him the most.