I think double speed has gone as a price plan. The idea was single speed rate limited you to 30mbps max. I have a phone plan that is before double speed, and a tablet plan from January claiming to support double speed. At Heathrow last week which is a 20mhz area (double speed) I was able to get 9meg download. Same in Nottingham last weekend. At home in a 10mhz single speed area I've seen 15mbps.
So as you can see its a marketing mistake to separate them, which I suspect is why since the website was relaunched I can't see much mention of double speed anymore.
James BT Infinity 2 19/09/2012 - Sold 42/6, Now 52/9, Sync @ 55 / 9.5 Mbps @ 470m approx
15 years broadband (1999 ntl: cablemodem, BT FTTC) - Router: Asus RT-N66U (merlin) - Modem: HG612 unlocked Typical speedtest