Sky Broadband is just slow - full stop.
They run a system which 'apparently actively monitors the line' and adjusts the speed to suite the line conditions.
This basically means they actively drop the connection speed over time, and use the excuse that it was poor line conditions.
I have two seperate lines into my house, both on new cable from within my house, to the street junction box. This junction box has also been replaced just weeks ago, and then connects to a new main above ground cabinet 200 yards away.
Because I am so far away from the exchange, my lines are only capable of supporting a maximum of 2.7MB download speed.
The line which runs my broadband is running happliy at 2.3MB, and never drops below 2.2MB.
The maximum speed the Sky line has ever reached is 1.1MB, and every time it gets ramped up because I complain, it then drops back down again, sometimes as low as 90KB DL speed.
I've ran tests on the lines and they are within 2% of each other.
Basically they are just throttling back the speed, as more people move onto the network, they should be putting some money into the network.