Royal Mail
The de facto postcode and addressing authority in the UK. Web site has Post Code finder by address or address finder via Post Code. Registration is required to use online service, though sometimes it is offered without logging in. Allows addresses to be viewed in standard mailing label format. Addresses in Wales are offered in the Welsh language.

Postcode Anywhere
Find postcodes or addresses in the UK. Regular usage requires registration.

AFD Software
Offers software solutions for finding postcodes and addresses.

Brainstorm - UK Post Code Search
Free postcode finder service by telephone number. Free downloads available of post code files and offers a postcode map of London.