I have encounted this problem before. I have previously flown direct from CDG on a QF codeshare to Singapore on AF metal. The position being (which was confirmed on here) that there is no lounge access in CDG unless flying J or F on the flight (ie no lounge access for WP).
For next flight in October, have re-rerouted on a QF codeshare (QF3625) on BA metal to LHR, then LHR to Singapore on QF. So to answer your initial query, yes you can get a QF codeshare from CDG to LHR on BA metal.
The benefit of this is at least lounge access in LHR before the long-haul flight. Can live without CDG lounge access before this flight.
My position is slightly different as flying Y, I think it may be different if you are in D, but CDG would have to be the most difficult airport to opine on lounge access.