You'll be way ahead financially if you reserve the option of flying INTO London (and perhaps taking Eurostar to Paris for fun), but DEPARTING from any station BUT London. The UK does not charge APD to arriving passengers but hits departing pax hard, especially when they're on premium tickets.

Urge flexibility, not only with the 2 X 2 strategem suggested by others but with arrival / departure cities. You can arrive at LHR, BRU, AMS, FRA, MUC, MAD and take a train or separately booked discount flight to CDG / ORY. Loosen up on that front and you have a far better chance of getting all four of you over there in business class.