Ah! Some channel (probably 5) did something like this recently. Hugely entertaining programme about people who proclaimed themselves to be 100% British (including the grotesque Garry Bushell and various other misguided, right wing numptys).
Their faces were a treat when it was revealed they were ALL at least part Asian/African/German/French/etc... The person that came out MOST British was some aristocrat whose family rarely left their living room, let alone the country!
Should be good...
Whilst trying to find details of said programme, I stumbled across this: http://www.peopleofthebritishisles.org/ - May be of interest...
G x