I could never find out if this was a common thing in the UK or not.
The ceremony is technically open to the public in any case, usually.
Less than 20% of weddings are religious (and a smaller subset of this will be in churches), and I don’t really hear of anyone just turning up at the ceremony of someone they don’t know.
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.