I have just checked on our ZS face lift ( which could be different on the MG5 ) but the chrome bright strips actors the top of the four doors ( that meets the window glass ) are magnetic ?.
If I remember correctly, the front grill on the Gen1 model, was plastic with a chrome(ish) finish laid on to the surround.
The chrome effect on the tops of the door handles are non metallic also.
My guess would be that the window trims would take something a light compound polish to remove the stains.
The problem here is of course, is that it tends to dry to a white smear on any type of black hard plastic or rubber.
You could mask off the rubber first I guess ?.
The front grill is a different beast really.
You would have to tread very carefully when using any type of abrasive polish on that fake chrome over plastic.