Commentary
News of Mozart's death reached London by 20 Dec 1791, when Haydn wrote his famous evaluation of the late composer's talent (Dokumente, 375). The earliest printed reference in an English source is apparently the above notice in The World of 21 Dec (first cited in Jones 2009, 155). Very similar or identical reports appeared over the next few days in the following papers:
22 Dec
London Chronicle (Nr. 5512); Gazetteer and New Daily Advertiser (Nr. 19668); Whitehall Evening Post (Nr. 6763)
23 Dec
Lloyd's Evening Post (Nr. 5380), Public Advertiser (Nr. 17930)
24 Dec
Times (Neue Folge, 76); Morning Chronicle (Nr. 7034, with Mozart's death "on the 15th")
The report was also picked up in:
Gentleman's Magazine 61 (1791): 1165; Annual Register or a View of the History, Politics, and Literature for the Year 1791, 48; Literary Magazine and British Review 8 (Jan 1792): 77; and European Magazine and London Review 21 (Jan 1792): 78.
Thus the news of Mozart's death received a very wide distribution in the British press. The idea of an "irreplaceable loss" may derive ultimately from a similar phrase in the announcement of Mozart's death in the Wiener Zeitung (Dokumente, 369).
The notice in the Morning Post (first cited in Jenkins 1998, 142) appears as part of general news from Vienna. It seems to be related to similar reports in the German press (see Neue Folge, 76). The "four pieces" ("four quartets" in the German sources) is probably a garbled reference to the three "Prussian" quartets, K. 575, K. 589, and K. 590. The report also appeared on 26 Dec in The Star (Nr. 1144) but with a later byline of 10 Dec.