Broken contractual obligations as well as health & safety issues are the order of the day at present. Additionally, like it or not the company have a duty of care towards employees particularly in the unique challenges of our profession. Don't forget; we can't even take a bottle of water through security!!
Assuming that a UK-registered aircraft is a place of work within the meaning of the health and Safety at Work Act, then the obligations placed on the employer by the Act include "An adequate supply of wholesome drinking water shall be provided for all persons at work in the workplace".
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HSE Frequently Asked Questions - Does my employer have to pr...
If EJ is indeed restricting the supply of free potable water, then someone with a direct interest should speak to their local HSE.
Until something happens on that front, is there any reason, apart from the time involved, to stop anyone wanting a litre of water airside making ten trips through security with 100ml each time? Ridicule can be the most effective way of fighting tyranny!