Join Date: Nov 2018
Location: Istanbul, Turkey
Programs: TK Elite Plus,BAEC GGL,ITA Executive, AFKL Gold,QR Gold,HH Diamond,Bonvoy Gold,ALL Gold
Posts: 14,186
Although you could pay any applicable fare difference between an IAD-LHR & IAD-LHR-CDG First ticket.
I know it might be expensive than purchasing a separate LHR-CDG ticket in CE but that's the only 100% guaranteed way that your bags would be checked through. Also, you would still get your 3x32kgs First Class baggage allowance.