I use Sonnet Internet.
www.sonnet.co.uk They have an 0845 number, never once had an engaged signal but it does cost £12.50 a month, but at 1p a min connect time it's good for what I want.
Sonnet is aimed at small/medium business and I've never had an issue with them in the 5 years I have been with them. Very easy going and tech support are excellent.