The study was the Progress in International Reading Literacy Study. The report is hard to wade through, because it primarily focusses on how US students compare to others.
>The Progress in International Reading Literacy Study (PIRLS) is an international comparative study of the reading literacy of young students. PIRLS studies the reading achievement and reading behaviors and attitudes of fourth-grade students {age is around 9 years} in the United States and students in the equivalent of fourth grade in other participating countries.
PIRLS was first administered in 2001 and included 35 countries, and was administered again in 2006 to students in 40 countries.
The countries are the US, 5 Canadian provinces, most of western Europe, some eastern Europe, Taipei, Indonesia, Singapore, Hong Kong, Israel, South Africa & Morocco, Iceland, a couple of middle eastern countries, and a few others. Nothing else in Africa, nothing in South America. NZ but not Australia.
Reading literacy is
>the ability to understand and use those written language forms required by society and/or valued by the individual. Young readers can construct meaning from a variety of texts. They read to learn, to participate in communities of readers in school and everyday life, and for enjoyment.
The study is
>PIRLS consists of two main components: (1) a literacy assessment administered to sampled fourth-grade students and (2) background questionnaires administered to students, their teachers, and the administrators in the schools in which the sampled students were enrolled.
Each country, province or whatever implemented the study based on procedures given them. Each booklet had the same reading passages. "Students who participated in the assessment received a test booklet containing two passages and were asked to answer a series of multiple-choice and open-ended questions related to the passages."
Between 2001 & 2006, average scores increased in Germany; Hong Kong, SAR; Hungary; Italy; the Russian Federation; Singapore; the
Slovak Republic; and Slovenia. Scores decreased in England, Lithuania, Morocco, the Netherlands, Romania, and Sweden.
In 2006, in all but two jurisdictions (Luxembourg and Spain), average scores for girls on the combined reading literacy scale were higher than average scores for boys. Surprisingly (to me anyway) the score difference between genders was highest in Kuwait & Qatar. I would have expected more boys to be highly literate in those countries.
Since the OP is about the UK, I'll add (BTW, it's listed as "England" in the study)
average number of hours spent on reading instruction each week
In England, 67% of students got up to 3 hours. 8% got more than 6 hours
In the US, 68% got more than 8 hours. Eastern European countries also tended to have 50% or more of students getting 8 hours or more a week.
Percentage of students who read stories or novels, by frequency of reading outside of school:
England: 33% "Almost every day", 17% never
US 36% and 19%, respectively
Italy 12% and 43%, the Slovak REpublic was about the same.
Most Canadian provinces had over 50% who reported reading daily.