Most pay-as-you-go ISPs give you a local number, which, depending on who your telephone provider is varies. One option is to sign up with Onetel on your fone line which gives you a special prefix number to dialout and they say they are 1p a minute. I have a couple of friends using this system and they tell me it 1p a minute any time of the day not just after 6pm.
Try their website. probably .onetel.com or .onetel.net not sure.