Of course we're not supposed to talk like that, are we? An ENP would be damned as bigoted, racist, and right wing before it had even uttered a word, whereas the other national parties would be principled upholders of basic freedoms fighting for their national identity
I'm guessing any ENP would be considered as you describe because that's what experience of English nationalist parties (EDL, BNP, NF, etc) has shown. Whereas these parties are invariably built upon racial lines, the SNP and Plaid Cymru purport to represent
all the peoples of their respective countries, regardless of nationality, colour, or religion.
And besides, the reason there has never been an ENP is simply because, with the Conservative Party looking after English interests, there has never been a need for one.