F.A.Q:
---What are the full titles of the various layers?
Layer names are abriged for compact controls, the layers full titles are:
OS (1888-1913): Ordnance Survey 6 inch to 1 mile Old Map (1888-1913)
OS (1955-1961): Ordnance Survey 1 inch to 1 mile Old Map (1955-1961)
LiDAR DTM 1m 2020:
-GreyScale Hillshade: LIDAR Composite DTM - 2020 - 1m - Hillshade
-Colour Elevation data: LIDAR Composite Digital Terrain Model (DTM) - 1m - 2020
LiDAR DTM 2m 2020:
-GreyScale Hillshade: LIDAR Composite DTM - 2020 - 1m - Hillshade
-Colour Elevation data: LIDAR Composite Digital Terrain Model (DTM) - 1m - 2020
While the "base" layer underneath the LiDAR and OS are open source streetmap and satellite projects.
Attributions for the various technologies and sources used can be found at the bottom of the map viewer.

---Why does the LiDAR have a grey layer and a green layer?
The gray layer is the result of a 'Hillshade' function, which produces a grayscale 3D representation of the terrain surface, with the sun's relative position taken into account for shading the image. Hillshading is a technique for visualizing terrain determined by a light source and the slope and aspect of the elevation surface. The Green layer is the true LiDAR data with colour helping to show elevation. The 'Hillshade' layer is better for general browsing as it loads faster but combining both layers produces the best clarity.

---What is the difference between the LiDAR layers?
The sets offer different resolution (how accurately the data is mapped) and coverage (how much of England was surveyed at this resolution) with details as follows:
The LIDAR Composite DTM 1m 2020 (Digital Terrain Model) is a raster elevation model covering >88% of England at 1m spatial resolution.
The LIDAR Composite DTM 2m 2020(Digital Terrain Model) is a raster elevation model covering >93% of England at 2m spatial resolution.