In order to certify and register religious buildings for worship and marriage you must apply to the Superintendent Registrar in the district where the venue is situated. There are particular signatories required in certain cases, and other conditions of application, but the Superintendent Registrar can assist with each application. Fees are currently £29 to register a building for worship and £123 to register a religious building for marriages. On the GOV.UK site you can view guidance notes and download an application form.
Superintendent Registrar contact details:
Wiltshire Registration Service
Tel: 0300 003 4570