I do not think the MOT specifies how the wipers are powered. If they did Ford Pops would be off the road! They need to work and leave the screen clear to see through. That is where plastic screens fail. Unless it is a wind defector -wind screen, clue was in the name, before the screen became a fixed ancillary, it will have to have an effective method to keep it clear. So a Messerschmitt sport does not have a windscreen. Its a wind deflector. So is a Brooklands style aeroscreen. A folding screen should get away with it too, but that is mostly Classic car territory and some are used in conjunction with Brooklands screens.
One of the few back dated rules is, if it has wipers, it has to have washers. But washers can be pipped down the wiper blade and not be that visible. On Schmitts some of us are in the habit of mounting the washer through the top right headlight adjusting mount bolt hole. Its virtually invisible, no extra holes. A Tunnock Teacake for the owner of every car where the tester squirts himself.