This question is primarily for Brits, although anyone knowledgeable is welcome to contribute.
Recently I conducted an activity at my school to spread a bit of information about common British slang. I selected the words to be used from a list found at: 88 very British phrases that will confuse anybody who didn't grow up in the UK
However, it was pointed out that my description may not have been accurate. I used it as an affirmative answer to a question:
Boy: Lovely weather today.
Girl: Innit? (Meaning: "Yes, it is.")
My co-worker informed me that the term most commonly is used as a question tag expecting a positive answer.
Boy: Lovely weather today, innit?
Girl: Right you are!
So, the question is: Can "innit" be used as a response to someone's statement with which you can agree, and are there any other usage notes that would be important to have?