There is a huge difference between a biscuit and a cookie. Just like between zucchinis and courgettes. Oxforddictionaries.com notes this subtle difference, giving two definitions for the word.
Short answer: A British biscuit is an American cookie and an American cookie is a British cookie and an American biscuit is a British scone and an American scone is something else entirely.
Long answer: http://tinyurl.com/pexlrrw
Source: Oxford Dictionaries