From here
http://www.tvlicensing.co.uk/about/foi-administering-the-licence-fee-AB20
Enquiry officers do not have any legal powers to enter your home without a search warrant granted by a magistrate (or sheriff in Scotland). They (like other members of the public) rely on an implied right in common law to call at a property as far as the door, while going about their lawful business and making their presence known. Enquiry officers must explain to the occupier of the premises why they are visiting, be polite, courteous and fair, and abide by rules of conduct.
You have no obligation to grant entry to an enquiry officer if you don’t wish to do so. If refused entry by the occupier, the enquiry officer will leave the property. If enquiry officers are refused access, then TV Licensing reserve the right to use other methods of detection.
Enquiry officers may apply for authorisation to use detection equipment if they are refused entry on to premises. TV Licensing may also apply to a magistrate (or sheriff in Scotland) for a search warrant. However, this is only done as a last resort and when a senior manager and a legal adviser considers that there is good reason to believe that an offence has been committed.