I say traditionally in the UK it has always been "jai" and that the American version of "Jay" has quickly taken over from it in the media.
Work colleagues say that "jay" has always been around in England and that "Jai" is just a regional variation.
Who is correct?
AB's typeface won't handle the characters, but the official phonetic pronounciation of the letter J in the English alphabet is represented by 'd3eI'.
(That's a 'd' combined with a symbol that looks like a number 3 with a dropped-down tail, and the 'I' is a small Roman capital 'i' ).
The 'd3' is pronounced as the g in 'edge'.
The 'eI' is pronounced as the 'ay' in 'day'.
This is complicated by Southerners tending to diphthongise the ay sound, and "cockneys" making it rhyme with eye.
Northerners pronounce it as jay, but with a flat ay sound, not diphthongised to ayee.