Prior to the destruction of the Second Temple in 70 CE, the most notable Diaspora communities of Jews were primarily found in regions such as the Mediterranean, including parts of Europe, North Africa, and the Middle East. Among the options you provided—Central Asia, Great Britain, Japan, and Southern Africa—Central Asia is the most plausible area for early Jewish Diaspora communities, particularly as trade routes connected various peoples and cultures. The other regions mentioned (Great Britain, Japan, and Southern Africa) did not have significant Jewish communities during this time period.
So, the answer is: Central Asia.