I could definitely see a movement like this using a lot of rhetoric and symbolism based on Arthur, Camelot, the Knights of the Round Table, the Holy Grail, etc. (Never mind that Arthur was a Celt who fought the Anglo-Saxons.)
British Israelism would also naturally lead them to adopt the Khazar theory or some equivalent that says that the Jews aren't really the descendants of the biblical Jews, which would both allow the British to have sole claim to being the chosen people and tie in nicely with the anti-Semitism that such a movement would inevitably be filled with.
Of course the various Celtic peoples (especially the Irish) would be a natural target for pseudo-scientific racism and discrimination, and if there's going to be a home-grown fascist genocide in Britain they would certainly be on the list of targets, as would Jews, Roma and Travellers, anyone non-white (other than martial races being kept for the service of the Empire?), etc. For fun, here's a quote from Benjamin Disraeli: ""The Irish hate our order, our civilization, our enterprising industry, our pure religion. This wild, reckless, indolent, uncertain and superstitious race have no sympathy with the English character. Their ideal of human felicity is an alternation of clannish broils and coarse idolatry. Their history describes an unbroken circle of bigotry and blood."