There's a pub in Sheffield that serves a beer so strong it's only served in one-third glasses. I think it's called the Rat and Parrot, can't remember the name of the beer. It was also used as a measure in Birmingham, I believe it was a called a 'stick' there.
I could imagine using a third for something strong (like that 9% Belgian I had the other day) but I couldn't see myself using it too much.
See you all at the Beer Festival.