That's going to be very difficult since there a so few dialup users left in the country now; I am assuming you want an 'unmetered' service and not a 'local call rate' one?
As I type this we are going through our entire ISP List database in preparation to launch the new listing system and I can tell you that a lot of unmetered dialup services seem to have vanished.
The review system has some reviews but they are mostly a few years old because obviously that section has not been active for a long time, due to the dominance of broadband. That and we launched the new review system when broadband was the main service.
The only ISP of any note that really still pushes dialup is Vispa, though Fast4 - despite a bad reputation for broadband - also has an unmetered dialup service still going. I think Vispa is the best bet but we cannot speak to the quality of these services anymore.
Mark 'Winter' Jackson
Editor-in-Chief - ISPreview.co.uk