Basically, the argument centres around the hideously limiting notion of the left-right political spectrum. It works well for the vast majority of political parties in any particular country (though not so well internationally) for the purposes of comparison of political ideology; however there are times where a party doesn't have a place in the spectrum because of an overarching policy that is in potential conflict with almost all other political positions.
The BNP have been placed at the far right of British politics by many of the other parties, notably the hard left groups of the Socialist Worker's Party and Respect. This is because they want to distance themselves as far as possible from the BNP, not least because many of their members are Muslims or otherwise ethnic minorities. The problem is although the BNP's major policy (white nationalism) is as far right as it gets, most of their other ideals (quoting from Michael Rock);
predominantly believe in nationalisation, collectivism and forbid free expression, which makes fascism the very antipathy of right-of-centre politics.
As I hinted to above, though, the BNP simply don't fit into the left-right spectrum.
Just my tuppence worth.
PS: For more information on other political spectra, take a look at this Wikipedia article.