As commented elsewhere, I'm just glad that despite the new rules I was allowed to drive Hesketh home to fit it, otherwise I'd be paying for mechanic discomfort at around £60 per hour.
A garage does not have the right to "impound" your car!
It could however get messy if you have to call the police for the return of your keys and property, as they could then do you for driving an unroadworthy car as you leave.
The garage must make it clear though that they advise against driving the car for safety reasons, otherwise they could get sued if you have, or cause, an accident after leaving.
I had a dealer try and stop me leaving when i enquired about replacing a broken tail light, but I had non of it and demanded my keys back.
(got home, and replaced lamp with cheap ebay one a few days later)