Have you tried changing the Input Menu to the British input? It is found in the International Preferences window. Only check the British input then uncheck the US input. Don't mess with he other input setting s because it's a bit tricky, too tricky, to reset them. This is when you're still can't rid of the dotted red line. Solution was probably mention earlier: Press the learn button as fast as you want to rid of the dotted red line in the Spelling pane. It is now become British English in your Mac😀
You could check the "Show input Menu in menu bar" option at the buttom of the Input Menu pane to remind you which input you use.