But if you’re seen drinking them by a steward they’d be within their rights to sling you out. More likely they’d just confiscate it but probably not worth the risk.
They don't have/need many stewards, so risk is minimal in surreptitiously decanting into the plastics provided, and there's no way they'd throw you out when they have a full day of taking your money. Worth it to avoid a couple of £5.50 Carlings!
But if you’re seen drinking them by a steward they’d be within their rights to sling you out. More likely they’d just confiscate it but probably not worth the risk.