The average point density for a survey depends on the required resolution of the surface model being produced from the point cloud.
For a 1m DSM surface model the point cloud will have an average point density across the survey of at least 1 point per square metre (ppsqm).
For 50cm this increases to 4 ppsqm and 25cm resolution is 16ppsqm.
It should be noted that the point density is calculated for the DSM (made up of the last or only laser return) and not the DTM. Under vegetation the point density that makes up the DTM will not necessarily have 1ppsqm at 1m resolution. The point density also varies across the scan - increasing where you have overlap from multiple flightlines and can also vary in forward scan direction in areas where the terrain changes significantly.
The point cloud is therefore not a regular grid and is why we talk about the density of a point cloud and not it’s resolution.
Access the Lidar data here: https://environment.data.gov.uk/DefraDataDownload/?Mode=survey