If you want to same money then try call18866 at:
http://www.call18866.co.uk/index2.php
Mobiles are 10p per min during the week and 2p per min at weekends, as far as I know there are the cheapest out. They also do local/national calls at 0.5p per min at all times
Oh and the sign up is free, and no monthly charges, just the calls you make.