I agree. Although I used to have Grammarly set to American English, about a month ago I switched it to British English.

For my longer articles, I usually leave the following private note:

"Note to editor: I have checked this with Grammarly. Any remaining issues picked up by Grammarly are ones that I don't want to change, so if you could please leave them as they are that would be much appreciated. Also, I have Grammarly set to British English, so please don't change words to American spellings. Thank you."

Also, you can edit an article after it's been published, so if an editor does change your spellings, you can always go and edit them back.