Words are not deeds. In published poems we think first of Eliot's "Jew", words edge closer to deeds. In Cline's anti-Semitic textbooks, words get as close to deeds as words can well get. Blood libels scrawled on front doors are deed.
In a correspondence, words are hardly even words. They are soundless cries and whispers, "gouts of bile," as Larkin characterized his political opinions, ways of saying, "Gloomy old sod, aren't I?" Or more simply, "Grrr."
Correspondences are self-dramatizations. Above all, a word in a letter is never your last word on any subject. There was no public side to Larkin's prejudices, and nothing that could be construed as a racist the word suggest a system of thought, rather than an absence of thought, which would be closer to the reality, closer to the jolts and twitches of self response.