This gist describes how to create GeoJSON polygons from UK statistical boundaries for use in choropleths.
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Download the boundaries you're interested in from the ONS's Open Geography Portal:
wget https://geoportal.statistics.gov.uk/Docs/Boundaries/Clinical_commissioning_groups_\(Eng\)_Apr_2013_Boundaries_\(Generalised_Clipped\).zip -O ccg-2013.zip unzip ccg-2013.zip -d shapes
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Re-project this from UK Grid Reference Eastings and Northings into Latitude and Longitude coordinates (i.e. EPSG:4326 aka WGS 84). You can use GDAL to do this: