If you know where (in which Registration District) the marriage took place - order from the local registration office.
Is that always possible? There was a time when local offices didn't have a unified index, so you had to tell them which church's register to look in before they were prepared to help.
Nevertheless, it's a useful reminder to have a look for the marriage at UKBMD (
https://www.ukbmd.org.uk/). It doesn't cover the country completely, and not many places have coverage as late as 2006, but you just might be lucky.
