You may have also noticed that Sky have been putting up banners on new shows being aired on Sky 1 SD, advising its customers (and having a sneaky swipe at non Sky customers such as VM) that the show is also being shown on Sky 1 HD. As well as this they seem to advertise new series as being on Sky HD and not mentioning SD.
Its obviously a ploy to upsell its HD services to its customers, whilst not telling them that it doesn't matter, the show is on Sky 1 SD anyway.