Let's keep it interesting and consider the position if you go the other way and fit steel wheels to a model which would normally be fitted with alloys...
Are they e marked?
You could certainly argue that the car is less desireable to steal. I would imagine in the event of a writeoff they would probably want the alloys back as they've insured the car on the basis that it's got alloys and when selling it for salvage they'll want the alloys which I think is fair enough. Thing is manufacturers can't legally have it both ways