The coin was struck by the Pobjoy Mint. It was struck to commemorate the 225th Anniversary of the British Captain James Cook taking possession of the island in the name of King George III; Cook took possession in January 1775. Captain Cook was responsible for naming the island, he did so in honor of King George III - hence, "Isle of Georgia" (later South Georgia Island).
In addition to the gold coin shown, the design was also used on copper-nickel (2 pounds) and 0.925 silver (10 pounds) coins.
The coins were non-circulating issues struck specifically for collectors.
(FWIW: I recognize that this is an older, dormant thread, but I just found this reply in my Draft folder. Apparently, I wrote it at the time of the original post, but forgot to post it. I figured that those who stumble upon the thread in the future via a search would benefit from a response to the OP's question. So...)
Collecting history one coin or medal at a time! (c) commems. All rights reserved.