Is dialog box correct in British English?
Yes, dialog box is the correct spelling in both British English and American English for the pop-up window that appears in a computer application to communicate information to the user.
For all other uses of the word, it is spelled dialogue in British English. In American English, dialog is an accepted (but less common) variant of dialogue.
The QuillBot Grammar Checker has regional language settings, so you can make sure you have the correct spelling of dialog/dialogue.