CARA's blog "1964" looks at the Catholic makeup of Britain in honor of the pope's recent visit:
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...
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 (an estimated 10.1% of those raised Catholic indicate that they no longer identify as such now).