This is a good question. The source is the "Survey of Adult Skills (PIAAC) (2012)." On the report they have a list of "National entities" (UK is not there) and

This is a good question. The source is the "Survey of Adult Skills (PIAAC) (2012)." On the report they have a list of "National entities" (UK is not there) and "sub-national entities", where they have Flanders (Belgium), England (UK), Northen Ireland (UK), and England/N. Ireland (UK). I guess they don't have data for Scotland and Wales.