Confirming what dracavia said is much the same for England and Wales. You are interviewed in person by appointment when you go to register your intent to marry, with passport or birth certificates needed to prove identity, and asked for details of your parents as well. Then you book a place and time for the ceremony where you sign the actual legal marriage part.

Someone would have to set it up very carefully and break a lot of laws doing so if they wanted to trick someone into a real ceremony that seemed fake, including having someone impersonate them for the intent to marry interview. Without that you'd never be able to book someone to perform a legal marriage ceremony.

Most local councils have their own sites with localised info on, but the CAB may have the most straightforward info on it if you want to look for anything specific here.