idnet are worth looking at if your seeking a cheaper alternative to  major suppliers.

All packages are on a 1 month contract, calls are  billed per second and rounded up to nearest minute and they only have UK based support staff.5p connection charge for non inclusive calls.

Line Rental starts at £12 a month but pay up front for a year and its £11 a month on average .( Weekend Calls Not Included).

You can also pay your call bundle up front.Line Rental and inclusive calls 24/7 paid that way works out at £16.50 a month.

http://www.idnet.net/voice_products/homephone.php

Call Rates are competitive : http://www.idnet.net/voice_products/callrates_packages.php