My understanding is that as long as an airline holds the slots, they can use them to whatever destination they like and for whatever equipment they want. So if the Air France group has some slots available (they will release three in mid-September and I do not recall that the Belfast slots have been replaced), they will have to use them at some point in order to grandfather them rather than seeing them handed over to BA or LH.
btw, is the rule really "no slots for props" or is it rather "no slots for 70 seats or less"? The Q400 is a prop and I doubt that LCY would want to rule that one out whereas 35 seat jets like the FRJ or ER3 would be allowed to serve LCY.