22 March 2018
According to a report by Stephen Bullivant, Professor of Theology and Sociology at St Mary's University, just under 10% of young people aged 16 to 29 regard themselves as Catholic. This compares with 7% who claim to be C of E and 6% Muslim. This is based on a European Social Survey conducted 2014 to 2016.
The survey is a snapshot, so does not strictly indicate trends. However, the fact that the figures are only for young people does indicate that within the next 15 years, there will be more Catholics and more Muslims than members of the Church of England.
The report does not mention that the survey covers the whole of the UK, whereas the C of E only covers England. I presume adjustment has been made for this.