One option is the long term car parks, where drop off remains free of charge, so you just park up by a bus stop and pile on with the bags. There is half way house: use the £5 drop off channel to drop the bags and sufficient passengers for check-in purposes. The car cannot be left unattended even briefly there. Then use the long term parking, or Hatton Cross or similar, to collect the passengers after check-in. 44 minutes in the Short Stay is currently £11 and you can leave the car unattended there.