The size of the Catholic population in Britain has changed little since the last papal visit in 1982 (growing only 2.7%). The most recent Vatican estimates indicate just fewer than 5.2 million Catholics in Britain (England, Wales, and Scotland). There is also now nearly the same number of Catholics across the Irish Sea in Ireland and Northern Ireland combined (the Vatican does not provide an estimate for Northern Ireland alone in the Annuarium Statisticum Ecclesiae).
Underlying the stability of the size of the Catholic population in Britain are indications of potentially significant change. A British online panel study (the same respondents are interviewed over the period of several years) conducted as part of the British Election Study (BES) series estimates that as many as 13 percent of those who self-identified as Catholic in 2005 no longer did so when interviewed in 2010. Most of those who left Catholicism (76%) did not join another religion and instead now report that they are not a member of any religion. Yet the panel study also shows that this changeover has had little if any impact on the size of the Catholic population because a similar proportion of respondents who were not Catholic in 2005, have converted (or returned) and now self-identify as Catholic in 2010. These changes are similar to the shifts identified by the Pew Forum on Religion and Public Life in the United States (the British survey only measure changes over five years, whereas the Pew survey measures changes over a longer time frame in reference to one's childhood faith. Thirty-two percent of Americans raised Catholic are estimated by Pew to have left the faith, representing 10.1% of the U.S. adult population. CARA's shorter-term estimates for change in the U.S. are available here).
According to the most recent BES 2010 (using in-person interviews), 9% of the British adult population self identifies as Catholic. The largest religious faith is Anglican (i.e., Church of England, Episcopal) at 28%. Yet, many respondents say they do not have a religion (47%; up significantly in the last decade). This proportion is much larger than estimates for the unaffiliated in the United States (Pew estimates this to be 16%).
Although fewer in number, Catholics in Britain are more likely than Anglicans to indicate that they regularly attend religious services. Thirty-five percent of British Catholics say they attend Mass once a week or more often and 19% say they do not attend weekly but go to Mass at least once a month. By comparison, just one in ten Anglicans attend services weekly and 13 percent attend at least once a month (all of these attendance estimates are likely overestimated due to social desirability pressures; see: The Nuances of Accurately Measuring Mass Attendance). The difference is very significant because it means that the number of weekly church attending Catholics (3.2% of the total adult population) is greater than the number of weekly church attending Anglicans (2.8% of the total adult population) in Britain.