Yes, focussed is correct in British English (and American English), but it is more usual these days for the simple past tense and past participle of this verb to be spelled focused. This is also true for American English.
While it is acceptable to use either spelling, it is best to make sure you only use one in any document to maintain consistency.
You can change the language settings in the QuillBot Grammar Checker to ensure you have the correct US or UK spelling of focussed or focused.