If they still want to call it a pint, they could go to the American definition (473ml, as opposed to 568ml). Beyond that, they could go to the New South Wales schooner (¾ of a pint, or 425ml). Or, you know, go metric and serve beer in decilitres as on the continent (400ml or 500ml is a reasonable size for a beer), though that may be politically impossible in the post-Brexit environment.