BA doing it without advising people beforehand is the issue I'd have, especially on long haul.

In CE I always take an old small Harrogate/whatever bottle and fill it in the lounge, at security it goes in the tray with the lid off to make it really clear that it's empty.

LHR security still sometimes silently steal my empty bottle, complaining is pointless as it's already in the bin before I realise, but it certainly puts me off taking a 'proper' bottle through there.

In F/J I don't expect the principle to be BYOB - if I'm paying BA a few grand, I expect the price to cover a bottle of water if I want one, that way I can wake up and sip any time I want, not have to call the CC for water and/or hope my glass isn't cleared away to avoid spills because I'm asleep.