The Master Map provides you with multiple levels of the Ordnance Survey (OS) map. Here's some further information:
The Master Map allows you to view a title and see how much of it has been built on, but showing you the building footprint. It’s also a great way to de-clutter the map!
You can access this view by clicking on the “Map Style” icon on the bottom menu bar.
This will then switch the view, and you can zoom in and out of the various levels of the OS map.
To change back to the Google Satellite view, simply click on the “Map Style” icon and select Satellite.