Where I've looked:
--Google: "big brother 2009 uk voting statistics" "how many people voted in big brother uk 2009" "big brother uk 2009 total number of votes" etc.
--Wikipedia entries for the series
--Big Brother website
--a couple of Big Brother fan forums - but I am not a fan of the show and I really don't know enough about the fan resources to find what I'm looking for
My impression is that the info is out there somewhere, since in the fan forums I saw references to voter turnout levels over the series, but none of the comments I found gave specific numbers or sources. I have found some stats for earlier series, but not for the one I'm interested in.