DTT (Digital Terrestrial Television) is being broadcast from all main transmitter sites and a good many transposers. Some or all of the multiplexes (each group of stations on one channel) cannot be broadcast at full power yet because of possible interference to existing analog channels. When analog shuts down in a region, it will be possible to bring the digital transmissions up to full power to increase coverage.
However, inability to receive some or all DTT channels by simply plugging a Freeview box into an existing antenna may be caused by other things. Were the analog signals already being received of adequate strength? If not, then there may be a problem with the antenna, and fixing it could result in acceptable DTT reception. In some locations an antenna of a certain size and at a certain height may provide passable analog pictures, but isn't good enough for acceptable digital. Replacing the antenna with a higher gain model and/or raising it higher may provide a solution.
There is also the question of aerial frequency groups to consider. The original UHF band plan for British TV allocated four channels within distinct groups for different transmitter sites, and antennas were optimized for the particular group. In some areas, DTT is operating on channels outside the original group, so adequate reception will require replacement of the existing antenna with a wideband type. (You can generally tell the antenna group by looking at the color of the insert on the end of the horizontal beam.)
So lack of DTT signals may be solvable. It all depends upon why the signals aren't adequate at the particular location.
