The first mention of Boris Uvarov in The London Gazette is the announcement of his naturalization on Tuesday, 20th July 1943 (#36101). The Gazette uses the 'z' spelling of naturalisation. Uvarov's Oath of Allegiance was taken on the 8th June 1943.
At some point, Uvarov acquired the post-nominal letters CMG as a Companion to the Order of St Michael and St George. The date given for this in biographies (e.g. Wigglesworth, 1971) is 1943, although I have as yet not found any evidence of this being published in the Gazette.
The final mention I have found in the Gazette is the promotion from CMG (Companion) to KCMG (Knight Commander) in 1961 (#42370).