The crew can check water quantities of water on the A320s. It could be topped up downroute subject to it being authorised if ever there is a need.
Many an airline have been flying for years without providing complimentary bottled water. I see no reason why BA would have a problem. If anyone is too precious to drink tap water, then bring your own or bring your credit card.
Gatwick is already more leisure centric than Heathrow. BA are simply responding to the market. I'm sure they have done their research and shock horror, they've realised people don't select flights based on a 'free' G&T and a packet of birdseed.
Out of interest do people who live in London all just drink bottles of Evian?
I see nobody has named an airline who charge different onboard prices based on routes so one can assume that no airline charges reduced prices on exEU flights.