Saint Helena is most known for being where Napoleon was exiled to and it has a larger population than you'd likely expect. (click here to read more about Napoleon). Today it is home to about 4,500 people and that number hasn't fluctuated much since Napoleon arrived in 1815. Napoleon wasn't the only one to be imprisoned on the island with the British using it to house approximately 5,000-6,000 Boers that were captured during the Boer War, c. 1900.
A volcanic island in the South Atlantic Ocean, it is one of the most remote places on earth. It is also one of the oldest British overseas territories with British rule dating back to the 1650s. The East India Trading Company had been granted a charter to govern the island however when Napoleon was made a prisoner there the British assumed control. After his death control was returned to the East India Company however in 1833 it was handed back to the British and the island became a Crown British Colony.
The island was used as a base for the suppression of the slave trade in the 1840s with tens of thousands of freed slaves passing through the island during this era. The island is well located to act as a gateway to Africa which made this a prime position.
In 1922 the uninhabited island of Ascension, remember the island where one thousand soldiers were stationed during Napoleon's imprisonment, was made a dependency of St. Helena. Following the war years industry suffered on the island with few options available for the inhabitants. It wasn't until 2002 that those living on the island gained British citizenship which has helped to raise the standard of living on the island as citizens are entitled to British assistance.
What about Ascension?
This tiny volcanic island is home to about 800 people today however the British appear to have put a cap on this number and you need written permission to visit the island (I haven't looked into this more but it seems likely given the island is dominated by a US airbase). Until 1815 it was an uninhabited place but when British troops were stationed there a settlement began. This grew as the island was used as a strategic base over the years both for trade and military purposes.
The island is said to be young geologically speaking with the undersea volcano creating the island just 1 million years ago.
In 2019 a report into the British overseas territories recommended that Ascension be administered independently of St. Helena as a territory in its own right however as of yet no action has been taken on this recommendation.
Without going into the story of the island in greater detail I can say it is an interesting place and I recommend this BBC article from 2016: https://www.bbc.com/news/magazine-36076411
Today both St. Helena and Ascension's population are descended from the early British settlers and Africans who arrived on the island via the slave trade. In 2017 an airport was opened on St. Helena opening up the island and the neighbouring Ascension to the world (prior to this a 2-week round trip voyage via ship was required and only one ship made the trek so it was a hard place to get to)!