Stella Rimington was the first female Director-General of MI5 and reigned from 1992 until 1996.
MI5 is Britain's Military Intelligence Section 5, their internal security service and MI6, the Secret Intelligence Service, is their external intelligence service. You could equate MI6 to America's CIA. MI5 does not relate direct to anything in America with the American Homeland Security coming closest. Britain has several other intelligence services similar to America with their various spook departments and FBI undercover operations but their is little direct correlation.