I have noticed that a bunch of ADM2 regions in the UK are missing population data. I am willing to extract and add this data where it's missing but I don't know the best place to look for it.
It seems to be the most recent census data available:
"The last census for England and Wales was on 29 April 2001. The next will take place on 27 March 2011 and will involve around 25 million households." (http://www.ons.gov.uk/census/index.html)