Looking at the IWF website. Only child porn is within their remit. This seems a little crazy breaking parts of the web and effectively assuming everyone visiting a file sharing site is a paedophile.
(I am not-dumpster diving for kiddie porn, however. It's a strict-liability offense: merely having the stuff in your hard drive cache, unlooked-at, is enough to draw a gaol sentence.)
I don't suppose the ISPs are too bothered about the sites being filtered - although there is some legal content on them, they're mainly used to host copied music and TV, so there is little incentive for them to make allowances for access to the non-porn content.
That's something that comes up periodically w.r.t. to the IWF itself. It's not an agency of the police (or part of the govt at all), so the exception to child-porn laws that permits police to view such photographs in the course of an investigation doesn't apply. Thus, if the law were to be applied as written, it should be strictly illegal for IWF employees to have accessed many of the sites on its blocklist. So either they haven't done so, and their blocklist is of questionable accuracy; or they have done so, and thereby committed a crime.
Fortunately for them, the government supports the IWF and has no interest in prosecuting them.
Incidentally, I'm with BeUnlimited and it seems uploading.com always thinks that my IP address has exceeded some download limit:
"Sorry, you have reached your daily download limit. Please try again tomorrow or acquire a premium membership."
This does not happen when browsing from a cheap Linux VPS box, looks like Be/O2 are proxying too, except from curl -I output it looks like it's a fully transparent proxy.