I've been with Zen for the last few years on the £24.99/512kb/uncapped download package for a few years now.
http://www.zenbroadband.com/ADSL.aspx?page=277
I was tempted to switch to Freedom2Surf - they offer 2MB uncapped download for the same price
http://www.freedom2surf.net/adsl/homeuser.php
Furthermore, they appear to be offering more 8Mbps LLU trials, with up to 768kb/s upload.
http://www.freedom2surf.net/adsl/press.php?release=31
than Zen, who limit themselves to Rochdale.
However, on looking on their respective support forums
http://forum.zensupport.co.uk/
http://www.freedom2support.net/forums/
it seemed that there were more unhappy campers on F2S than Zen, with people concerned that F2S are now owned by Pipex.
According to ADSLGuide, Zen appear to consistently outperform F2S, which in turn outperforms Pipex
http://www.adslguide.org.uk/isps/compare.asp?f2s=ON&pipex=ON&zen=ON&cmp_action=Compare
In the end I upgraded to the Zen 2Meg package. It may be £10/month more, but worth it for the peace of mind.