"You may take your vehicle to get it repaired after is has failed its MOT test but you cannot use a vehicle with dangerous faults, or you can be fined £2,500, be banned from driving and get three penalty points. If it has major faults you can drive it to get repairs, but if the police stop and deem it unroadworthy you could be prosecuted."
IE If you get a fail that is deemed "Dangerous" on the fail certificate you are committing an offense if you drive it away. (Not sure if Garage can effectively confiscate vehicle or if they are under any obligation to grass you up)
Under any other circumstances you can legally do so, but you are not immune to prosecution if the Police want to give you a hard time.
(Just thought this was slightly clearer
)
Now, Just a thought, Can you lawfully "Tow" a car with dangerous faults?
IE if you fail on a "Dangerous" are you committing an offense if somebody recovers you using an A frame or tow-bar?