|Someone wrote in miss_s_b,|
You need both to legally drive on the public road if you car is between 3 and 25 years old (unless you are taking it to the garage to have it MOTed). You can't get it taxed without the MOT certificate and there is a fixed £60 fine if you get caught without an MOT.
If you don't have these then most likely your insurance is invalid as well.
Also - I think that simply to be *parked* on the public roads you need both (might be wrong about this though).
From the point of selling it: having an MOT is the absolute bare minimum that any sane person would demand before buying a car of that age except at a stupidly low 'go on I'll give you 50 quid to take it off your hands' price.
Sorry not to be more optimistic despite my thinking this road-tax is generally bad idea thing. [Got no objections to the MOT concept - cars are dangerous, polluting things even now: without MOT they would...well, 'Be unsafe at any speed']