The amplitude of seismic waves decays with distance as they travel through the Earth. This means we can only detect smaller earthquakes at shorter distances. Also, we need to record the earthquake signal on a number of sensors to make sure it is "real" and not just local "noise".
Assuming that we need at least four stations to record a signal that is twice the background noise level to be able to detect a seismic event, then we can calculate what magnitude of earthquake we are able to detect in different places for any set of seismic recording stations.
The UK seismograph network is designed to detect nearly all magnitude 2.0 earthquakes in mainland UK. This means we are usually able to any earthquakes that are felt by people.