Hi @Bunnster
I find the streaming quality information on the Youview Boxes when using the NowTV App is all over the place and fluctuates between 128, 3800, 4000 and 7800 for me with the HD Boost Pass.
My internet speed is a stable 39Mbps with no other devices used in the house whilst watching on my BT T4000 Box.
I mainly notice the dynamic change of the streaming rate on the Entertainment pass when watching a HD 1080p TV show though the picture still looks like full HD to me, where Sports & Movies for me usually remains with a figure of 7800 when i press the information button on the remote controller.
A similar question was also asked on this thread linked below.
https://community.nowtv.com/t5/Set-top-boxes-Smart-TVs/BT-Youview/m-p/519541#M9621