The availability grids in Lidar Download show all the OS tiles where there is some Lidar data available, however the actual Lidar data may not cover the full tile. For example you may download a tile of Lidar data that only contains a small area of actual Lidar data, and the rest of the tile is blank/empty. The tile below shows the OS 5km x 5km grid in red with a tile of Lidar DTM data beneath. Note that there is no Lidar data for the south east corner of the tile.
The two images below show Aberystwyth harbour, boats, cars, and buildings are all visible in the DSM on the left, but these features are absent in the DTM, shown on the right hand side.
The data is provided in the British National Grid projection system but is not supplied with projection files so you may need to manually set the projection in your GIS/CAD application.
Links to a number of online tutorials and resources are provided below:
In Lidar Download the Show Availability button on the Product Information popup allows you to see the extents of each dataset:
The availability grids can also be turned on/off through the Show Availability Grid panel on the right side of the map window.