10 Downing Street: Prime Ministers in History: Official UK government site provides profiles of all prime ministers since Robert Walpole in 1721, including portraits from the Government Art Collection.
Archontology: British Prime Ministers: 1730-2004: Biographies extracted from Dermot Englefield, Janet Seaton and Isobel White, "Facts About the British Prime Ministers: A Compilation of Biographical and Historical Information" (1995). Each has a portrait or photograph.
Britannia.com: Britain's Prime Ministers: Short biographies on every British Prime Minister from Walpole (1721) through the present.
Spartacus Educational: Prime Ministers 1715-1990: Offers biographies of British Prime Ministers from Robert Walpole to Margaret Thatcher.
The Hewett School: British Prime Ministers: A list of Prime Ministers from 1721 to today; includes some biographies.