I would send in a very short complaint which leaves out all of the side issues such as who got apple juice.

Simply say, "I was distressed that BA did not load sufficient water for my flight from LAS-LHR on [date] to serve all passengers. While serving me, a FA declined to provide a glass of water for my husband. This is a health & safety matter far beyond customer service."

I can absolutely understand the liquor thing on LAS flights, it's a problem for every foreign & domestic carrier serving the airport. But, not restocking potable water on a flight which is essentially TCON+TATL is foolish. Water is also handled differently than alcohol and, presuming that the aircraft is catered at all which it is, there's no reason for the water issue.