Here you see the LPD entry for niche. My poll shows that the French-style pronunciation is now overwhelmingly preferred in BrE, while (as far as I know) remaining virtually unknown in AmE. The OED comments
N.E.D. (1907) gives only the pronunciation (nitʃ) /nɪtʃ/ and the pronunciation /niːʃ/ is apparently not recorded before this date. H. Michaelis & D. Jones Phonetic Dict. Eng. Lang. (1913), and all editions of D. Jones Eng. Pronouncing Dict. up to and including the fourteenth edition (1977) give /nɪtʃ/ as the typical pronunciation and /niːʃ/ as an alternative pronunciation. The fifteenth edition (1991) gives /niːʃ/ in British English and /nɪtʃ/ in U.S. English.So it’s BrE that’s done the innovating here.
In one of my talks about pronunciation preference polls I show three slides to illustrate the point that the evidence is that — contrary to what many people suppose — in some cases change spreads from AmE to BrE,
in other cases from BrE to AmE,
while occasionally we find that both are moving in tandem.
(Sorry about the Comic Sans. This is a very old talk. I’ve moved on typographically since then.)












