The celebrant can legally perform a wedding service including the exchanging of vows and rings but the legally binding part of the ceremony can only be carried out by a registrar employed by the local authority in a venue officially licensed for weddings. 

Many people are now choosing to get married by a registrar privately and then have a celebrant to perform a more public ceremony. This means that they can hold the wedding wherever they choose, e.g. at home, or in an outdoor space, and they are not as restricted as to the content, (registrars are not legally permitted to include any religious references, hymns etc).