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Infobel
Find anyone anywhere in the UK. This website includes comprehensive updated White and Yellow pages for all United Kingdom. It is a free service that allows nationwide search throughout the UK.
in English

192.com
Directory for UK. Free search allows 20 searches per month for registered users (it's free and takes only 30 seconds) but it returns results not just from BT's directory enquiry database but also from the fully comprehensive UK electoral roll as well as business databases. The information is tied into aerial photographs and street maps for the whole country.
in English

PhoneNetUK
Official British Telecom White Pages for the UK, includes both people and businesses. Covers England, Scotland, Wales, Northern Ireland and the Isle of Man (but not the Channel Islands). You need to specify area, nationwide searches are not available.
in English

Ask Alex
Fast and effective listing of businesses in the UK, has some nice map links and other resources.
in English

ThomWeb
Thompson Directories, hosted at Infospace. Fast, efficient searches for business phone numbers. Can search entire country by business name, or can limit search by category, county, town and postcode area.
in English

Scoot
Business directory. Search by category/location, or business name/location. Includes UK, Isle of Man and Channel Islands. Seems rather incomplete.
in English

Scoot People Finder
Residential listings. Requires free registration. Appears to be for England, Scotland and Wales only. Returns people "near" a location, often a totally different town.
in English

Electronic Yellow Pages
The UK's electronic yellow pages from British Telecom. Allows searches by start of business name, as well as by classification. Search with a location of "UK" if you are unsure.
in English

The Freephone Directory
Directory of freephone (toll-free) numbers in the UK.
in English

GT Online Telepages Directory Enquiries
Directory of people and businesses in the Channel Islands of Guernsey, Alderney and Sark. (Does not include Jersey).
in English

UKPhoneBook
White pages directory for people and businesses in the UK. Same basic data as PhoneNetUK but has nationwide search and more complete addresses. Has links to maps.
in English

Colour Pages
Business directory for parts of London only. Has map links.
in English

UK White Pages
UK residential listing, from the US company Infospace. Not from official sources (British Telecom have the copyright on the listings in the UK). Many people have mentioned that it doesn't seem very up to date or complete. There are many misspelt names, and duplicates, and other errors, use with caution. Listings of both businesses and people.
in English