........
I knew if the police were to see me when driving away from the MOT garage I would be breaking the law. .....
You've got to be able to leave the test station to either take your car home or elsewhere for repairs. 
But not
before the Test Andy. You have to prove to the police that you have the car booked into a garage for repairs / MOT, and which garage that is. If, as I was twice, going
away from that garage that had the booking for both, I could hardly claim I was doing anything but driving a car without an MOT, and without a legal reason.
Yes, I could have lied and said I was going home, but as I had gone onto the M20 which I do not have to do for the registered address for my vehicle, then I would have had a very awkward situation! Of course, even then I could have been allowed to go back towards the garage, but the law is strict on that, and if the PC is a stickler for "the word" of the law, as most must be, then I could have been in trouble. They could have also said my insurance was invalid in those circumstances.
In short you are taking a risk by deviating from the exact word of the law and I have seen what happens when I have been out with police patrols.
