• Residential numbers
• Business numbers
• Reverse lookup

Residential numbers:

BT:

• BT Phone directory         The most complete, with good wildcard search options. Use an * asterisk to leave a name or city incomplete: i.e: James Smit* or Cambr*

192.com

• 192.com       One of the best resources to find someone in the UK.  You have to register(for free) Some services are available only for a fee. 
• 192 People Finder Using this page you can search over 200 million records from the 2002 to 2007 Electoral Rolls, 13 million people in the Directory Enquiries database, 3 million Director reports and over 300 million Birth, Marriage & Death records from 1837 onwards.
• 192 Electoral Roll  Current and historical Electoral Rolls from 2002 onwards
• 192 Directory Enquiries Free records, updated daily. Residential & business numbers.
• Advanced People Search Find people by family members names or other co-habitees that live at the same address. Allows you to specify any type of location details or no address at all
• 192.com Family Records Searches all UK Birth, Marriage & Death records from 1837 to 2004 plus 1861 census information
  
• 192 Aerial PhotosStreet maps with every UK street labelled and the highest quality aerial photos

Others:

• Infobel UK    Good search tool, easy to use results, plus a map
• InfoSpace's UK listing        Good interface, results not always reliable
• Scoot    Register, then it's free
  

Business numbers:

• EYB Yellow Pages       Official BT site. Excellent way to search by name, location or industry
• Scoot        Cuter than EYB and gives you 
• Infospace Business Finder        Includes search by name and category
• Ask Alex         Promises 1.8 million businesses. By name, not location
• InBusiness         Search 2.2 million companies

Reverse United Kingdom:

• 192.com        For a fee, a reverse search via telephone numbers and street address
• Advanced search    Simply put in an address or a post code and find someone in a house or everyone on a street.
• 192 Electoral Roll   
• 192 Directory Enquiries
• 192 People Finder

JNet's Main Phone Book Page