Sky does not yet offer mobile phone services, but they do offer broadband and content services.
Virgin Media offers a quad play, with content, broadband, telephony and mobile services.
What these companies have which no mobile operator (including TalkTalk and BT who both own IPTV services) is content services.
Mobile networks were, a few years ago, seen as dominant, with new streams of revenues stretching out over the horizon, but short sightedness, the predominance of walled gardens and an under-estimation of the threat from competitors has seen mobile networks becoming totally commoditised.
Now, unlike in the most of the rest of Europe and the US, mobile operators are running shy of content services, and TV services in particular. Frankly, it's too late for them and something very radical, such as a merger of BT and ITV, would be required to change this.
It looks like a brave new era of broadcasting will be dominated by BSkyB and the BBC for the foreseeable future.