In weddings in the U.K. (or at least in England) anyone can attend a wedding - legally they have to be open.
It’s therefore not uncommon if it’s local for more distant friends of family, neighbours, etc. to pop along to the ceremony at invitation of the couple or their parents as a result, but not to be invited to the party part. Sometimes older guests will just come to ceremony too.
That's an insane legal requirement. I'll do the legal wedding in the most unceremonious, quickest manner possible, and then have my real ceremony in privacy and not tell anyone about it.
It's largely historic these days, there's been various proposals to reform it and other mad rules (e.g. can't get married outside, can't get married after 6pm) but it's not really viewed as high priority.
It’s therefore not uncommon if it’s local for more distant friends of family, neighbours, etc. to pop along to the ceremony at invitation of the couple or their parents as a result, but not to be invited to the party part. Sometimes older guests will just come to ceremony too.