I used to use Freenet when I was on dial-up they used to have an 0800 number but they changed recently to an 0845 number which is charged as a local rate call, so you only pay while your on, I think this is the cheapest you'll find (hope someone proves me wrong )
How to connect to Freenet:
1. Create a new dial-up connection to 0845 662 2000
2. Enter your username: freenet
3. Enter your password: freenet
4. Now you can connect!