Something like that is only useful if you're outside the car, attempting to swing a bat/wrench inside the car would be useless. The guy you're trying to hit would probably end up taking it off you and using it on you to better effect!
I do, also, whats wrong with bolting it to the throttle body?
Bolting what to the throttle body?
I thought keeping a tyre wrench under the drivers seat would not be thought of as keeping a offensive weapon as its there to remove a wheel in the event of a puncture?
Seems pretty clear cut to me:-
http://www.bsdgb.co.uk/index.php?Information:The_Law_Relating_to_Self_Defence
Internet bravado kicks in once again.