A natural condition of any MGA, roadster or Coupe, is to have lower air pressure outside above your head than pressure inside the cockpit (when the car is in motion). For a roadster this leads to air being drawn upward out of the cockpit from an area aft of the windscreen to about the back of your head (which is why you don't get wet while driving in gentle rain). Air then comes into the cockpit from the back, so your shoulders might get wet if it rains hard enough. It also tends to blow your hair around from the back, which is why "windblockers" have been gaining recent popularity.

For the Coupe, if you crack the vent window open an inch or two it will exhaust air in a big way due to the lower pressure outside just aft of the canopy front corner. That can bring cool air through the heater box onto your feet. This works well if it's raining and you want to keep the side windows closed. Turn the vent windows all the way around to serve as air scoops, and air will come in ahead of the glass and exhaust behind the glass. Crack open the side windows at the same time, and you may get air inlet both in front and behind the vent window glass as it exhausts out the side.

If you want open a sun roof (article AT-207) to get rid of hot air, you need to consider where the cool air will enter. With side windows closed the only obvious vent entry point is the heater box. If you don't have a heater or fresh air vent system, or if you close the heater intake vent, then the roof vent won't do much more than cause wind noise and maybe a little turbulence around the top of your head. I suppose air would have to enter at the sides and exit out the back of the raised panel. But crank open a side window and you get much better results with the open roof panel working like a vacuum cleaner to pull air through the cockpit resulting in good heat exhaust up high and likely some nice ventilation around your head.

If you want to install some cockpit exhaust vents other than the sun roof, they need to be located near an outside low pressure area. Modern cars may place such vents in the rear door jamb where air exhausts out the slot between door skin and rear wing. This would work in the MGA, but it would be only a small vent area (good for winter heating with closed cockpit). Another good location for an exhaust vent would be in the rear post of the roof between side and rear glass. It would also work with a vent just below the rear glass or in the rear body tonneau aft of the rear glass. In any case you need to include a baffle and some means of water drainage from the vent port to keep water out of the car when it rains.