Bt have finally admitted there is a problem
"Wed 29/07/2015 at 10:26
BT customers are having trouble loading webpages"
All other service incidents prior to this are unrelated.
And it's not Windows 10 - I turned that shit off, have not seen any unexpected downloads on my SNMP graphs, and bandwidth is not the issue. It's packet loss or a routing issue (secure/encrypted connections seem to fair the worst - https, rsync over ssh etc. both failing).