The British used concentration camps in South Africa during the Second Boer War (1899-1902).

To give the background, after the British colonial administration in Cape Colony made increasingly threatening encroachments upon the Boer republics (Orange Free State and South African Republic (also called Transvaal)), these states marched armies into the British colonies and laid siege to a number of settlements. After reinforcements from the UK reached South Africa, they made a slow yet powerful march into the Boer states and seized their capitals, driving the Boer forces into the countryside.

These Boer combatants weren't soldiers, mind you. They were mainly farmers, who lived on the land in which they now fought and knew it well. They were good shots, due to hunting, and often armed with good, modern weapons like Mauser rifles. And they had horses, which meant they could fight an effective guerrilla war against the infantry-based British armies by striking with hit and run tactics.

The British then had to reflect on how to fight the war after conventional strategy (seizing key cities) had not forced the desired surrender. With the capture of these cities came a change in command, and Lord Kitchener took control of the campaign. He decided to implement three main strategies: firstly, to establish a series of blockhouses linked by lines of barbed wire like a web across the countryside to hinder Boer movement, secondly to concentrate the local non-combatant populace in locations where an eye could be kept on them, and thirdly to implement a scorched earth policy to deny resources to their enemy.

It's up in the air as to the main reason why these camps were set up. Pakenham's book suggests that the camps were initially set up as refuges for pro-British civilians to escape possible persecution by the Boer guerrillas, but then as the scorched earth campaign burnt countless farms and homes (both pro-Boer and pro-British alike) their populations swelled with the influx of now-homeless and starving civilians.

Pakenham also details that these camps (which were run by the military) were poorly administered and terribly unhygienic, which resulted in large numbers of deaths amongst their populations. A report was made on the camps by Emily Hobhouse, and this prompted a commission to be sent out under Millicent Fawcett to investigate. The result was that the camps' administration was taken over by civil authorities and, belatedly, conditions began to improve.

His actions in the Boer War are a big part of why Kitchener is such a controversial historical figure, as he achieved military successes in the name of empire but he would at times employ what the Liberal leader, Campbell-Bannerman, would describe as 'methods of barbarism' to do so. The Boer War is a prime example - he implemented the concentration camps but his methods did eventually force the final Boer surrender.