Well to be fair the number broadcast IS the starting point but it can be affected by many things. Interference, poor aeriel, badly positioned, poor cable / plugs/ using wrong multiplex, interference, faulty equipment Etc.
Also some channels like RED Button are not listed. All I am asking is how many people actually get. Looking at this website I see there are approx 133 channels in Total http://en.wikipedia.org/wiki/List_of_digital_terrestrial_television_channels_%28UK%29