IIRC,
You can drive a car with no M.O.T. or Tax to and from a PRE BOOKED M.O.T. Testing station without breaking the law.
You should drive directly to and from the test station using the shortest route too you.
The car MUST be in a road worthy condition though, so normal things like lights and tyres should be in good condition, otherwise you may still be liable to a traffic offence.
The only thing you can work with would be the statement, M.O.T. station should be the nearest (within reason) to your home

If your buying the car from Plymouth! thats not near your home

so book one in with your nearest M.O.T. station by your home
Make sure you advise the M.O.T. station that the car will be driven to the test without TAX and for them to make a note of it, in case they are contacted by Police for conformation.
Make sure your journey time is the shortest necessary for the trip to the station
The best course of action would be to contact the local constabulary for the areas you are driving through for there stance on things