Affect on network bandwidth
Ol' Grumpy: "In BT's case, they also claim they put a Quality of Service policy on the router so the home network always takes priority over the public one and therefore your service shouldn't be impacted."
"shouldn't" is I guess the operative word. Only if transactions were instant would this be the case. Due to the latency of the link, once a data stream has been established, packets can continue to appear from the remote end for a relatively long time even if the local end tries to put a stop to them, potentially saturating the download path and so interfering with downloads the high priority user wishes to make.
Of course, how much effect this has in practice is open to debate.