The approval of a merger between Carlton and Granada TV franchises creates a monopoly in mainstream terrestrial TV advertising. This can be seen as a response to a declining market, as advertising revenues were declining in any case. In competition terms a merger will have little impact given the low amount of competition there was in these regional franchises on advertising rates in the first place.
The real issue is whether a merger will create more competition between terrestrial and satellite television advertising, or with other forms of media. As satellite and cable head towards 50% of viewers you can understand the feeling of regulators that a merger in a declining market may help prevent a monopoly in a growing segment. However, a structural solution may still be required. Most advertisers have alternative (and probably more effective) means of advertising than television, which over time has become increasingly less effective. The exception to this, and the only growth area in television in recent years, is government campaigns and public information broadcasts.