If you work in the motor trade, you're likely to need road risk insurance unless there's no need for you to drive vehicles as part of your day-to-day. However, this is very unlikely.
Even if you're driving a customer's vehicle to simply test drive any new repairs you've completed, the correct motor trade road risk insurance needs to be in place. The only time a car doesn't have to be insured is if it's been declared SORN.