My understanding is that draft (draught) beer in the UK must be served in one of three measures; 1/3 pint, 1/2 pint, 1 pint. Glasses must be certified either to the brim (no line) or have a line denoting the proper measure. Other than the Great British Beer Fest (GBBF), I have never seen a 1/3 pint glass and CAMRA only introduced that glass a couple of years ago. Some pint glasses may have a line at 1/2 pint and then either a line at a full pint or be brim measure.
CAMRA has made a push for oversize, lined glasses but the industry - at least around London where I've frequented - has shied away from them. As it was once explained to me, some pubs - Wetherspoons were one chain IIRC - tested the oversize glasses but many patrons asked for "top-ups" (a fair request for a brim measure glass but not for oversize glasses if there's beer to the line). The staff were often unwilling to make a scene so would accommodate which resulted in lost revenue. Unscrupulous patrons learned they could get an extra few ounces by making such a request. The pubs switched back to brim measure pints.
Another reason for lackluster adoption of the lined glasses has been that the pubs (and suppliers) have a large stock of brim measure glassware which could be costly to swap out. Sure, new lined pints could be brought in to replace breakage but that could take a while for a full swap out and then there's the issue with use of two glass types (see above) in the interim.
I personally like the line glass. A lined glass at shows a given volume with room for head (or not). A consumer know whether or not they've gotten a full pour and what that amount is/should be. Some US pubs (Gordon Biersch, Rock Bottom Mug Club mugs are two off the top of my head) use these. The standard US "pint" sleeve/mixer glass comes in too many variations, not all of which actually hold 16oz to the brim. Some pubs are using imperial pint glasses but there still is no indication to the consumer whether they're getting a proper measure. I'd guess if the US had more enforcement with regards to ensuring a serving called a "pint" actually held a pint, menus would just change to calling it a "glass" of beer.