In answer to Fenwar:
As I understand it, the problem is not that Sky owns a lot of TV channels as such, more that Rupert Murdoch owns Sky. He also owns a lot of other media companies (not least 20th Century Fox, The Sun and The Times), so (theoretically) is in a position where he has an awful lot of control over what an awful lot of people see and hear.
People (at least in part) base their descisions on who they are going to vote for based upon what they see and hear.
Now, I don't know Murdoch's personal views, and I don't know how much influence he wields over the electorate, but if you look at the political bias shown across all his media companies, it is fairly consistantly tory (even though the Sun is currently supporting our government, our government seems to have more in common with the tories than labour).
The rules were put in place to ensure that no one person is an a position to influence a large chunk of the population. Buying ITV would put Murdoch in a position to influence over half the population of the UK.
Richard Branson and Virgin (all the virgin companies) don't have nearly that much power, and haven't shown any willingness to use whatever power they do have.