I don't understand exactly what you're looking for.

Domain access count ranking data, processed by the attached script, in exactly the format specified :-)

Is it just a list of the most common sites UK residents are visiting?

Effectively yes, but to a depth that is not available commercially ie accurate down past 100,000 which requires a userbase into the 10s of thousands.

How would that help a filtration program?

In and of itself it does not. What is does do is provide an ordered list of important domains where important means people really go there and their browsers actually request pages from those domains. We have a dozen guys/gals that do little else but poke these domains into categories - currently we have a million domains classified into the 100 cats the filter uses. From the filtration point of view knowledge of frequently visited sites has an impact on the performance of the product. Having a classification for http://please.someone.visit.my.site.com/ where no one ever goes is a waste of review effort and makes no difference to the end user experience.

Why would an ISP turn over such data?

In the past it has helped to be good friends with the sysadmins ;-) The crisp folding stuff has also been known to tip the balance.

It is rather broad and I can't see any real privacy concerns with it, but that won't be everyone's impression.

The data is distilled to a form that is totally anonymous in that there is no user information or even specific page info retained. The lawyers say this does not breach the Data Protection Act. Funny how you have to pay a small fortune to get to the same conclusion you came to in a few seconds.....but that's lawyers for you.