The Motorplan issue is very open ended. Some dealers when you call them will tell you it's a void issue. But in practice it's normally not the case.
Most of the dealers will say it has to be approved BMW tyres, whether it's RFT RSC or normal tyres. (Star Rated)
I've fitted non approved non RFT tyres with no issue.
If they do want to cancel it they will only cancel the affected area ie wheels+drivetrain, diff gearbox clutch etc...
You'll get lots of confusing opinions non of which are wrong, but practice shows that you can do it and just be happy
As you can see many here have posted so already.