I was looking to see if there are exceptions to the Weights and Measures requirements for a Plimsoll Line or brim of glass. But the posts were intriguing on several levels.
First, when the craft beer movement began and the straight pint glass adopted, IIRC it was a Libby glass and actually only held 15 ounces; the movement wasn't strong enough, yet, so Libby wasn't going to produce a true pint to order.
Second, the English pint id 20 U.K. ounces, which translates to about 19.2 USA ounces ( the ounces are not equal).
Third, when I visited England in 1976, we went to a "local", the White Swan. They had tried to adopt a Greene King Abbot glass with a Plimsoll Line; the locals hated it, thinking they were being cheated. So, the landlord gave me two of the lower-half-dimpled mugs, still one of my all time favorite beer glasses.
Finally, fourth, visiting friends in Yorkshire about 20 years ago, we went to a busy, real ale pub, which had on hand pump Timothy Taylor's Landlord, a legendary best bitter on my bucket list for donkeys' years. That part of Yorkshire wants a good head, but real ale has only a small amount of almost imperceptible carbonation. As the bar maid pulled the first half pint, the sparkler tried to agitate the beer. She pulled another half pint, filling the glass, then pulled again and again and again, the ale spilling over into the drain trough (I was told the beer goes down and back into the cask). She handed me the full pint and I walked back to our table. The beer was cloudy and my local friends said I should take it back. One taste and I was more than satisfied the beer was in good nick. Over the next five minutes, the cloudiness gently lifted, leaving a fine ale with a good head and perfectly clear.