To create a new Datawrapper click here. Select "Choropleth Map" as your map type. When selecting your basemap, a search for "United Kingdom" or "London" will bring up the three map types that are relevant for the referendum:
1. United Kingdom » Unitary Districts
2. United Kingdom » Electoral Regions
3. United Kingdom » Greater London
Click "Proceed" and you can start adding data to your map. For more information on how to create choropleth map, check out this short tutorial on Datawrapper Academy.