Update : Now it is signed by Verisign -- it was briefly signed by 'British Telecom' -- which was untrusted. I have a .PNG of this if anyone wants to see it.
Are you on BT Broadband, or using one of their hotspots? Because you probably got redirected to their "You ran out of money, top up your internet!" or "Uh oh! Your router disconnected, please wait until we're back online" pages.