If you weren't insured then you weren't insured. Also generally working for a garage and having the relevant business insurance means that you're insured to drive cars you're working on and not any car ever.
You may well lose your license. Sucks I know, but powering insured. It's the law that you should make sure you're insured before taking a vehicle out. Therefore even in court you'll be guilty.
If your boss admits that you were told you were insured and you checked that this covers you to drive any car, then you might be able to get off. Sorry man.