As far as I'm aware, BA crew on WW (not sure about EF) are entitled to 2 small water bottles. If they don't drink them in-flight, then they are allowed to take them off with them. I've been doing it for years without being challenged.
This also applies to uneaten crew food. It is yours to eat, either on or off the aircraft. I would always clear out the crew cart (left over crackers and chocolate bars, etc) on the shuttles from ANU and take them back to the hotel for the crew to share at the room party. Again, it is yours to do what you want with. Passenger food, including left over Raid the Larder doesn't apply and would be constituted as theft.