A lot of 'Americanisms' are actually of English origin, having survived export whilst losing influence in the UK. Similarly with words like 'sidewalk' and pavement'. Sidewalk is English in origin, pavement French. As the song goes, you say 'Tomato' with an 'A' as opposed to an 'AH'. When British started colonizing America a lot of English in the provinces would have pronounced it the American way. It was in London and the upper echelons of society where Received Pronunciation was the norm, that the 'AH' sound in tomato spread to the general UK population. There's a really good piece on The Great Vowel Shift here :
https://www.youtube.com/watch?v=VOOAb7erAmE