I'm not sure about this. I just joined sky for a backup line and I got the internet for free for 6 months (only paying line rental) and then £7.50 a month for unlimited.
So was it misleading? My first bill supports their claim! Not sure how unlimited it is as I only have 11mbps but I can still shift some data I am sure!
In fact here's my bill - am I looking at this all wrong? (they have been true to their word on the phone when I ordered it)
https://imageshack.us/a/img266/8602/qxm3.jpg