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.
Just to update this old thread, Zotero 6, released earlier this year, includes additional spellchecking dictionaries, including U.K. English. It can be configured by right-clicking on the note editor.
https://forums.zotero.org/discussion/19771/workaround-to-disable-spell-checker
(about:config is accessible through the advanced tab of the Zotero preferences)