There are many TV transmitters in the UK, but only 81 of them carry all the services including the three commercial multiplexes. The other 1,036 transmitters will only carry the three public service transmitters, sometimes called Freeview Lite.
As the transmitters can cover millions of homes down to less than 100, this means that:
In England, there are 54 full service transmitters and 583 public service transmitters, but the three-multiplex transmitters only serve 7% of the population.
The main reason for the commercial operators not extending their networks is the uneconomic nature of these sites. It is not simply a question of being allocated frequencies by Ofcom (these were freely available to the operators) - the cost of installing the three antennas on each mast, the considerable capital expense of providing the transmission equipment and the ongoing network, power and monitoring costs.
In Scotland, the 14 main transmitters cover 85% of the population with full Freeview, but those in the mountains and islands are again uneconomic. Indeed, the Black Hilltransmitter alone covers 40% of Scots.
Looking to Northern Ireland, the Divis transmitter provides services to 67% of the population, with two other full service transmitters providing the next 13%. However, again 20% of the population are provided by the other 43 and these will provides just the public services.
TV viewers in Wales stand the most chance - one in three - of being short of channels. Nine main transmitters cover 66% of the population, and in the mountains and valleys the other 196 transmitters cover very small populations each.
Map showing public service only areas in England, Wales, NI and Scotland.