A certain sort of idea of Europe, or the West, or "the white race" provides a foundation for Fascism. A wildly unrealistic foundation, but so what?
A certain sort of idea of being English, or Scots, or Welsh, or Irish, or Ulster-Scots, or Celtic, or Anglo-Saxon, or Norse provides a foundation for Fascism. A wildly unrealistic foundation, but so what?
And these two foundations are wholly compatible with each other.
Neither, however, is remotely compatible with allegiance to the United Kingdom.
The limited appeal and ongoing collapse of the BNP are therefore entirely to be expected, and wholly in keeping with the history of such phenomena.