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You didn’t mention whether you were able to download the British English dictionary.

If you weren’t, do so via File > Options > Corrections > Spelling > Download:

But a few of us are unsatisfied with the lack of words in the dictionaries that Scrivener provides. If you find yourself in that camp, you can download a more robust one.

See this thread for a detailed discussion of the topic, where this post contains a link for downloading a few “levels” of English dictionaries, British included.

And see this post in a different thread for instructions on where the Scrivener dictionary folders/files are located and how to replace them.