You can't switch to UK English, but you disable it
https://forums.zotero.org/discussion/19771/workaround-to-disable-spell-checker

(about:config is accessible through the advanced tab of the Zotero preferences)

I also find it rather agonising to be bombarded with red coloured lines under correctly spelt words in my notes and wonder whether UK English might be an option one day. I have clumsy typing fingers, so do find red underlines useful and would prefer an alternative to the workaround linked above.

Same.