I also had the water problem last month out of LCY on BA1 to JFK.
Something to do with contamination of the supply at LCY.
Initially we were told that the toilets were out if service between LCY and the stop in SNN.
Then just before the doors closed, a ground handler got on board with a dozen large bottles of highland spring water and the toilet embargo was lifted.
Really enjoyed the LCY to JFK service. Had a great chat with the Captain during the flight who stepped into the galley for a cuppa and a chat with a few PAX.