As a larger ISP, Plusnet would have had more customers, and hence would be worse effected (in the sense that more people would have had problems) to BT outages.
Regarding the balancing/subscription, as the outage happened, many would have done reboots, and I assume (someone more in the know can confirm of contradict this) Plusnet's networking would have allowed some of them to connect via other nodes/gateways, resulting in higher use of said nodes/gateways (less balance). Once the outage was rectified, the load balancing would have started.
Here's a graph showing Wednesday night's latency on my connection. Notice that the latency on the original link is far higher throughout the evening. Most likely due to more subscribers on the gateway (but again, I'm open to correction from others more in the know).
I'm all for having a pop at a supplier if they are at fault, but I don't think this specific instance is one of them.
~ Camieabz ~
All Connection Data ~
plusnet
mod'er·a'tion n.
Synonyms: temperance, restraint, modesty.