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 -
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:
ogr2ogr -f 'ESRI Shapefile' epsg shapes/CCG_APR_2013_EN_BGC.shp -t_srs EPSG:4326 -
Simplify the polygons (to make your map load and render more quickly) using a tool that keeps the topography intact (i.e. that will simplify the boundaries together - if we did them one by one leads we would be left with holes and intersections). Mapshaper is great:
mkdir boundaries mapshaper -i encoding=ISO88591 epsg/CCG_APR_2013_EN_BGC.shp -simplify 5% -o format=geojson boundaries/all.json -
Split the polygons into a single file per boundary so that they can be served and styled separately. It's stretching it to call this a ruby one-liner but it will do the trick. Note that you'll need to specify the key name of the gss code (e.g. "CCG13CD"):
ruby -rjson -e 'JSON.parse(ARGF.read)["features"].each{|feature| File.open("boundaries/"+feature["properties"]["CCG13CD"]+".json","w") {|f| f.write(feature.to_json)}}' < boundaries/all.json