The cost to the employer is not relevant for taxability or not - it's the benefit to the employee (a "benefit in kind") which is.
Here's the facts, straight from HMR&C web site
The benefits code: cash equivalent of benefits: the general rule Cost to the employer it is.
I daresay the Revenue do review BA regarding this and I'd assume it's done on an overall not individual staff member basis.
Meals on board - flights are generally overcatered to allow passenger choice - staff get what's left, so no cost. As for the complimentary washbags, staff are not allowed to accept them.