That won't stop you car getting nicked. I'd think about it logically, are you taking all reasonable steps to prevent unauthorised access to your firearms by leaving it in a car, on a public road, while you are elsewhere at work?
Plus, are you willing to risk your rifle and licence based on the opinions of people on an internet forum?
Unless you live so far from work, that it is totally impractical to return home to collect the rifle then go shooting, I personally wouldn't even contemplate it.