This seems to be the most interesting finding of the respected CARA institute at Georgetown University in Washington. In 2008, the Catholic populations of Britain and of Ireland were roughly the same: 5.2m in each case.
The study also confirms that, despite being many fewer than Anglicans, there are Catholics in church to Sundays than their Anglican counterparts -- 35 per cent of Catholics go to Mass once a week or more.
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.Here is the relevant table: