It looks like there was quite a lot of logging on at work for BBC coverage of the Olympics. Auntie reports 68.3m unique browers from the UK for Olympic content over the two weeks.
BARB puts the 16-and-over population at 59,210,000, and 4-15 years at 8,931. Ofcom figures say 81% of the UK population has fixed or mobile broadband.
So either many more people have more than one 'device', or occasionally use somebody else's connection.