You get it from
www.sky.com. It's treated as a package upgrade. I signed up this way and all BT checkers say the say the same thing for my line (yes to 0.5Mbps, maybe for 1Mbps, > 1Mbps for ADSL Max). All you have to do is go to sky.com, click on MySky and log in (create a profile if you don't already have one). You can then do an upgrade or click on Sky Shop. For me, I was given the choice of base, mid or max when I entered my phone number. FYI, I get between 4 and 4.9Mbps connection speed.
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Phil Chung
Sky Broadband Max