Westminster Libraries - Your local, free, public library service. Includes details of each library, services available and access to the online catalogues.
The British Library - The website of the UK's national library, with access to their catalogue.

WorldCat - Search a variety of library catalogues from across the country selecting either the whole country or a particular area e.g. Greater London then stating where you are; it'll find where the book is closest to you!
COPAC - Merged access to the catalogues of the large research libraries in the UK & Ireland.
LibDex - Searchable international directory, with links to web-based catalogues where available.

M25 Consortium of Higher Education Libraries - Details of over 160 college and university libraries in the London area, with access details and information on their stock, including searchable access a union catalogue of university libraries in London.
SCONUL - A scheme whereby students from one UK institution can use the library of another, if their 'home' institution is a member.
CILIP - The professional body for library staff in the UK (formerly the Library Association). Includes careers advice for library and information workers and job vacancies.
DCMS - Government department responsible for public library policy in the UK.