You're missing the point, this is an incremental USO. You have to assume each time the USO increases BT will have a maximum of 1-2 years to meet that or face fines.
That's difficult to achieve in a 2 year timeframe if you don't roll out full/pure FTTP to every line over 500m, if the Government's current goal is ubiquious/blanket 100Mbps+ ultrafast broadband .
As said, the devil is in the detail.