P.s if our bedroom caught fire, I think the wooden furniture, the cotton bed clothes and woolen carpet would be more of a concern than a 40cm PU ceiling rose dripping on us should we happen to stand directly underneath it. Thanks though
I fully accept that I may be dense, but why can't you do what most people do and screw the plastic rose to the surface (the visible side) of the PU rose. That is what most people do. They drill a hole through the PU rose wide enough for the cables to pass through, and then reconnect them in the plastic rose.
In cases where the new light fitting doesn't use a plastic rose, they use connector blocks/WAGOs etc to connect the wires up and push them into the hole in the decorative rose.
Is your concern based on aesthetics (ie you don't want to see a plastic rose) or a worry that you won't we able to wire up the rose correctly afterwards?
I typcially put the electric ceiling rose ontop of the decorative ceiling rose.
(ie Ceiling, decorative ceiling rose, electric ceiling rose)
Long screws from electric ceiling rose, through the decorative ceiling rose into the ceiling.
I do not counter sink the electric ceiling rose into the decorative ceiling rose