O2's broadband is provided by Be (
www.bethere.co.uk) - they bought Be out not so long back.
They can offer 'up to 24Mb' but only through enabled exchanges (less than half of the total I believe) and I don't think they are enabling any more.
As with all DSL products, your very much limited by distance from the exchange... on the 'up to 24Mb' products, you need to be very close to the exchange to have any hope of getting it, we're on be at work and getting about 15Mb with the exchange literally over the road.