Actually any ISP that is not affiliated with Tiscali aka Dynamic DSL, aka Pipex will at least not block your connection to WinMX, however, most of UK ISPs do throttle in some way or another, so much like at election time its the lesser of two evils we seek.
The best ISP I'm aware of right now in the UK is called BeThere
. This is only available in some areas tho so visit the site to check on your area. As of this time no reports of throttling in any way shape or form.
After that Id have to say either Virgin or BT are the next best, with semi sensible throttling of speeds during peak times for heavy users (based on usage per month).
Carphone Whare house is worth a mention here as the second worse in UK history, since they purchased AOL they have recently taken up AOL's former throttling ways leaving peoples WinMX connections nearly crippled far worse then Dial Up speeds.
Aside from whats mentioned above I cant comment on so much, SKy have some issues, Talk Talk also have some issues throttling, but all of these are better then Tiscali if your a WinMX User.
Hope these help ya and do let us know.