At that time in the UK, it would probably have appeared easier to try to persuade on other issues a party already hostile towards Jews than to try to create hostility towards Jews among other parties. BDS and the associated leftist hostility towards Jews did not exist back then (BDS was only founded in 2005). In conjunction, Savitri Devi already knew she did not have long to live (she died in 1982, the same year the BNP was founded(!)), hence could not expect to have many years to spend waiting for a better organization to come along, but had to take her last shot with with whatever was immediately available at the time. In light of this, her association with the BNP is understandable. My best guess is that she had hoped that the fledgling BNP would incorporate some of her ideas in order to distinguish itself from the NF (from which the BNP was splitting), thus at least creating a comparatively more ethical party to rival the NF.
With that said, Savitri's grasp of Aryan racial theory was very messy. She never provided a central definition of the term in her own writings, but made only peripheral judgements on who was or was not Aryan based on throwaway criteria (mostly while writing about other subjects!). While Savitri and I might agree on an individual historical figure's Aryan quality, we are more likely than not to differ in where we guess their blood came from. For example, Savitri credited Akhnaten's Aryan blood to alleged Hittite ancestry, whereas I credit it to Puntian ancestry.