Easiest way to customise your map is to use the Map Content panel to the left of the map when in the Digimap Ordnance Survey Roam application (digimap.edina.ac.uk) For many views you can switch layers on and off using the check boxes next to the features in the legend. If there are no check-boxes then you will need to change the zoom level or the basemap.
Either zoom in or out or click on the Basemaps tab at the top right of the map and select different, non-raster map product.
Have a look at the how to use Roam help page: http://digimap.edina.ac.uk/webhelp/os/osdigimaphelp.htm#roam/how_to_use_roam.htm
There are sections on Controlling map content and just before it a line about changing basemaps.