This particular key responds directly to the locale (phone’s selected location) that the phone is set to, regardless of the language model/layout in use. So if your phone thinks it’s in the UK, you’ll get a £, if it’s in the US, you should get a $ (and Yen in Japan, Euro in France and so on).

FYI there is a multiple currency select key for the most common currency symbols on the symbols layout, and then again as individual keys on the second symbols layout.