Coming rather late to this but...
NaviExtras is the method by which the pre-facelift MG5 navigation is updated.
As I understand it, the facelift MG5 has a completely different method. I think, if you go to this page and log in with your car's VIN number you can find details about updating the maps etc.
HTH.
Still strange to note UK customer service team does not know the difference in upgrade process between Older Exclusive and SE windows based infotainment system and the Facelift android based infotainment system.