Based on our experience in Europe in 2022, they may keep chasing you for flight details (although that was for Heathrow which has multiple terminals). There was absolutely no problem just booking it as transport one way only and without a flight to get, but I suspect the system flags missing flight details and on the cruise we kept getting messages on the app asking us to provide them. I nipped to Guest Services one day and explained we had a car at Heathrow and our onward journey was driving back home and they did some magic against the booking and we got no more messages.