The term "Scientology International Headquarters" is ambiguous, as at any one time there have usually been more than one "International Headquarters", and the nature and responsibilities of these "headquarters" change over time.
At the start of Scientology in 1953, there was no international structure.
A bit later, there was "HCO WW", i.e. Hubbard Communications Office World-wide.
From the mid-1960s to early 1980s, the two principal "International Headquarters" were
- the Sea Org-operated management network, run for many years from a ship named the Apollo, then from Los Angeles, and
- the Guardian's Office (GO), based at Saint Hill, Sussex, England.
The GO usually handled principally external affairs, but also involved itself in some internal Church matters such as finance.
Starting the early 1980s, several additional International Management organizations were created, including the Religious Technology Center (RTC), Watchdog Committee (WDC), Office of Special Affairs (OSA) and others. There are those who argue that some of these organizations are not part of the Church, but in reality they run the Church. OSA took over most of the external affairs from the GO when it was disbanded in the early 1980s.