From here:
The law only allows you to use reasonable force in self defence. So unless the aggressor is carrying something that can be used as a weapon, you're not allowed to use that bat. e.g. If a person comes up to in a threatening manner, but they appear unarmed, you're not justified to use your bat -- if you do, either your attacker or the police can charge you for assault, even though you didn't start the fight.
Even if the other person presented a weapon (e.g. a tyre iron) and you use your baseball bat in defence, the police might ask you why you had a baseball bat in your car. Some martial artists recommend that the best 'weapon' to carry around are everyday objects that would seem reasonable to carry around regularly. A cricket bat or a baseball bat is an odd thing to carry around unless you're going to/from training... a better object may be a golf umbrella.
I keep a golf umbrella in my car -- not to use a potential weapon, but just in case it rains! And even if it's a blazing hot sunny day, we know that the weather forecast isn't always right. I actually have one umbrella in my car and one at work (because unless it's raining when I get to work, I usually don't take my umbrella out of the car, so I have a spare umbrella next to my desk in case it rains after I've left my car and I'm at work (cos it's a fair hike from my desk to the car park -- all uncovered outdoors).