You don’t have to follow a legal procedure if you want to start using a new name. However, you might need a deed poll to apply for or change official documents, like a passport or driving licence. Provides a link to free forms allowing you to do this.

Information on how to change your child's surname. 

Information about where and when to register a birth, what information you will need to provide, and who can register a birth. Also includes information about the possibility of adding the father’s details at a later date by completing an application for the re-registration of a child’s birth.