Yes it is related to my request. Project-GC's code largely gets its data from the OpenStreetMap data. It appears that the datasets there that are being used (which appears to be using the tag Tag:boundary=administrative admin_level=6) correspond to local government unitary authority areas in England & Wales and twenty year old extinct regional government areas in Scotland.
My initial investigations suggests it might be possible for them to use the OS Boundaries - although that requires using a different API so understandably they are reluctant to do that. There might be another alternative though the OSM data has a tag boundary=historic this appears to be used for boundary data where the historic boundary still has meaning today. Counties seems to fit this perfectly.
The question then is how is it possible for the OS Boundary data that, shows the historic counties currently in use throughout Great Britian, to be loaded into OSM. There are probably all sorts of licensing issues aside from the practical element of someone actually populating the dataset.
However if it was achieved it would put to bed the huge anomalies that UK users suffer with "counties" that bear no relation to what people anywhere in GB think of as counties.
I'm not sure how best we could get the data that exists from the OS as open source into OSM tagged appropriately. It almost certainly requires someone with sufficient practical experience of adding OSM data to be involved. There are also lots of hurdles in terms of making sure that adding such data is acceptable to the wider OSM community. There are articles discussing this on the OSM wiki from 2007 so its not a new consideration.
So the practical upshot is it isn't possible at present but could be possible in future it just needs a shove from someone with solid OSM experience.