They are a rep...
This is an old debate, IDNet do not have capacity for all users to make 100% utilisation of their connections at the same nano second is my understanding, but that they monitor usage and make sure their actual capacity on the network they directly control and own always has a large degree of headroom.
The £10 priority is for priority just across the Openreach/BT Wholesale segment, and may or may not make a difference. Experience over the years suggests it can make a difference for mild congestion in the local area but as usage continues to grow your performance can still be impacted.
For a business with servers on the IDNet network this sort of traffic behaviour will be very good though, for consumers making much more random extensive use of the internet at many locations, it is likely that the wider internet contention will be a limiting factor.