^Well an shop on eBay sell many issues of UK Vogue, in the March issue from 1986, on the info said than Beatrix was still the EIC, not Wintour, i think the first issue of Anna as EIC of the mag was the july issue, in the same year with Christy Turlington as the cover girl.
And also on the british Vogue's website, in the cover archive (1984 to 1986), appears the name of Beatrix Miller as the EIC (except the july issue, with the name of Anna Wintour), i don't have an idea when the Liz Tilberis's mom leave the occupation.