Spelling bee is a game of two hives

Can anyone shed any light on a strange cultural diaspora. We get it that the ‘humor/humour’, ‘aluminium/aluminum’, ‘sulfur/sulphur’ etc schism now exists between the predominant forms of English. My question is does anyone know if and when there was some kind of pronouncement from either country, or has it just grown up in a custom and practice fashion (though I probably mean practise!)
The reason I ask is that in doing some British Library research I discovered that many 19th century English newspapers right up until the 1870s at least, confidently and continuously use what nowadays Britsnobs would consider to be crass Americanisms. Was it one of those cul de sac movements (like the now deceased fashion for the internationalist language Esperanto) that we should all coalesce around the simpler, more logical US orthography. And was it the reactionaries in Britain’s university senior common rooms who reverted to throwing in extra letters again?