I think there's some misinformation here. BT shares it's Retail Broadband Network at the national level between business and consumer broadband users. In the business day, very few consumers are online so business gets pretty much the full capacity. Since consumers heavily outnumber business users this allows really good business response throughout most of the business day. As consumers start using the service - starts picking up from 4pm - the network gets busier. It's congestion, rather than active shaping.
Contention ratios at the exchanges went out years ago. The key change was the increase in capacity of the circuits between exchange and core network. This effectively removed the bottleneck and the need for contention ratios. So it doesn't matter from that point of view.
Of course, there's always the chance that you get local interference from 4pm ish. The consumer traffic build should be gradual. If your speed goes off a cliff it might be interference of some sort.
