The cause of these issues could be any one of many things, which is even why I havent jumped in claiming network congestion (and a few on here know I tend to do that).
Modern isp networks are more complex than they were say 10 years ago due to the need to filter out certian types of traffic, this means they are scanning traffic, of course in plusnet's case they also run their own advanced QoS kit. This means there is additional things to go wrong.
Their could be a bottleneck on a DPI device, such i/o or, cpu which restricts tcp stream performance at busy times.
Could be their QoS actually throttling traffic as its determined it needs to for whatever reason.
Could be network congestion.
Could be a bug in firmware/software.
Could be a BT wholesale problem.
Could be a misconfiguration, obviously increased complexity increases the chance of this happening.
Given that plusnet cannot do ipv6 on the BNG equipment, I have a gut guess that this is hand me down equipment from BT branded broadband services that has been deemed no longer suitable for the modern requirements of BT broadband.
Because plusnet are now quite secretive we can only speculate.