Spell check is using US spelling not UK spelling like the rest of office and there is no way to change it
Teams spell check should be able to use UK dictionaries
Richard Jennings commented
As Microsoft is based in US, they think there is only one version of English; theirs. Need to support the languages actually installed on the computer, in my case English Australia which is based on English UK.
Or any other language spelling.
It would be good to have non-US spelling for system messages as well - e.g. favourite
Sean Ellis commented
Should use selected system dictionary by default, with option to override for specific conversations. (You may be chatting to someone in Spanish in one tab, in English in another.)