The bnp defines british as
“The expression Indigenous British, as understood by the defendants [the BNP], appears to mean the descendents of persons settled in these islands between the end of the last Ice Age, about 11,500 BC, and 6 July 1189, the date of the accession of Richard I, when legal memory began. This may possibly include the descendents of Roman legionaries from Africa and elswhere outside the boundaries of Europe”
I don’t think many people can record their family history this far, are we all suppose to take dna tests now to see how british we.