We’ve had laws requiring landlords to serve standardised pints the UK since 1698. This is not a new problem.
Agree this is a fact - we mark our glasses because of it. https://www.gov.uk/weights-measures-and-packaging-the-law/pr...
And they say England becoming low-trust is a recent change !
I like the CAMRA ones which have the pint line below the rim of the glass. Because technically the head doesn't count as part of the pint and there's a lot of back-and-forth about what the legally acceptable amount of head really is (CAMRA say 0%, pub associations serve 5%, pubs themselves serve whatever they think you'll pay for).
1985 is the current one: https://www.legislation.gov.uk/ukpga/1985/72/contents
It's why all bars will have a little plaque saying what size their shots are. Almost always 25ml these days, but 35ml was common in many places. You're allowed to serve shots of either size but not in the same pub.
( edit: Better link: https://www.gov.uk/weights-measures-and-packaging-the-law/sp... )