> Less than 20% of weddings are religious (and a smaller subset of this will be in churches)
That's a likely a fair underestimate because many religious marriages aren't legally valid because of various requirements that the Church of England doesn't have to follow as the state church. In Catholic churches for e.g. they need to register the building, then either appoint the priest as an authorised person or get a registrar to come to every ceremony as in a civil wedding. They do usually do this but most non-Christian religions don't bother with this at all and so the couple end up just having a civil ceremony first and the religious one after.
That's a likely a fair underestimate because many religious marriages aren't legally valid because of various requirements that the Church of England doesn't have to follow as the state church. In Catholic churches for e.g. they need to register the building, then either appoint the priest as an authorised person or get a registrar to come to every ceremony as in a civil wedding. They do usually do this but most non-Christian religions don't bother with this at all and so the couple end up just having a civil ceremony first and the religious one after.