Just another update for you, yes you know who you are, (imaginary friends are so fickle if you don't mention them)
Anyhoo,
Although it's only been a few days since BT did some backhall work, it seems (not much data yet) that while there is improvement, its not really that great. maybe the peak contention has just moved up the network a bit, or the ISP is still oversubscribing there I/O paths
I don't really think that a 20Mbps is acceptable for the main hours of usage between 19:00- 23:00, not when you have been sold 80/20 and you know none peak is stable well above 50Mbps for as long as you like. (usually 74Mbps for me)
As for that 20Mbps, that's kind of a hard floor on the test the past 48hours, no sure of the reason for that... please speculate.
As an aside, this is what downloading a file (UE4 game engine in this instance) is like during peak, and offpeak