There are a few ways in which it's measured. The Ministry of Justice publishes statistics, broken down by department, to say how many requests they've had—and what percentage has been answered, what percentage is delayed, and what percentage has not been disclosed. That in itself acts as a kind of lead table for performance, and the media and others comment if a department isn't doing particularly well.
Recently, the new information commissioner in the U.K. has created a new list of bodies that he is watching. When a body performs particularly badly according to different criteria, such as delay, he puts them on the list for six months, and claims to be watching them for signs of improvement, and will keep them on a kind of “naughty list” until they do rectify their behaviour. There are elements of public accountability and also accountability by the regulator.