Among the South African, Palestinian and other young exiles debating revolutionary politics on campuses across early 1980s Britain, there was little at first to mark out Riek Machar, a twentysomething student from what is now the troubled young country of South Sudan.

Yet within a few years – while pursuing a philosophy PhD at Bradford – he was to establish an underground student grouping in contact with rebels in his homeland and lead a delegation to Muammar Gaddafi’s Libya on behalf of the Sudan People’s Liberation Movement (SPLM). Distinguishing himself as a field commander during one of Africa’s longest-running conflicts, Machar formed a new and more personal relationship with Britain in 1991, when he married Emma McCune, a young English aid worker who subsequently died in a car accident in Kenya.

Nearly three decades on, Machar, a former vice-president of South Sudan, is a rebel leader in a brutal post-independence conflict in which both insurgent and government forces have been accused of atrocities.

The legacy of those British links, and those of a generation born in Sudan when it was still a British protectorate, endure. Machar, other rebels and senior government figures are all UK citizens, having taken the option to upgrade their status from British protected persons (BPPs) – a fact that human rights activists say places an unique responsibility on the UK.

“If British citizens are suspected of involvement in some of these atrocities, the UK should certainly do its bit to ensure they’re not in any way shielded from justice,” Amnesty International told the Observer.

Redress, an organisation campaigning on behalf of victims of torture, said the 2001 International Criminal Court Act gave Britain jurisdiction to prosecute war crimes allegedly perpetrated with the involvement of UK nationals abroad.

Last week, international development secretary Priti Patel said after a visit to South Sudan that the killings and other atrocities there amounted to a genocide. This was a step up from the UN’s warning that a genocide “could happen”.

“It is tribal – it is absolutely tribal,” she said. “So on that basis it is genocide.”

Now a new chapter is opening in Britain’s involvement with a territory it once governed. In the coming months, hundreds of British troops are to join an initial contingent already there as part of the UK’s single largest deployment to a UN mission and a major re-engagement in peacekeeping.

Back in Britain, meanwhile, members of the South Sudanese diaspora are looking on with increasing exasperation as the death toll rises from ethnic strife and a human-created famine. Benjamin Avelino, chair of one UK-South Sudanese community organisation pushing for Britain to take a more “forceful” approach to promote peace and rights, said the British citizenship of those he described as “despoilers” of the peace process could be used as leverage.

“They are here or travel here frequently. They have property and – I’m saying this because I’m desperate and we have had enough of these people – they come for treatment when they are ill,” he said. “In some case there are those who still have council accommodation.”

Facebook Twitter Pinterest Machar with his wife, the British aid worker Emma McCune, who was killed in a road accident in 1993. Photograph: STR New / Reuters/Reuters

In South Sudan – where the UN last week said a further million people are on the brink of famine after warning in February that 100,000 people faced starvation – the conflict breaks down largely along ethnic lines. President Salva Kiir, of the Dinka tribe, has been pitted against Machar, a Nuer, since accusing him of plotting a coup in 2013.

But a UK government source familiar with Britain’s engagement in South Sudan pointed out what they believed to be the difficulties of using British citizenship of protagonists as leverage, taking the case of Machar. “He’s a British citizen because he qualified to be one. The British government can’t take responsibility for the actions of its citizens in quite the way people would like it to. If he asked for consular assistance, we would provide it because he has a right to it, but we do nothing else for British citizens when they travel.”

Nevertheless, the source said that Britain’s influence on both sides was strong and it was well-regarded for the “honest broker” role it played when a 2005 peace agreement with South Sudan’s former Sudanese rulers in Khartoum was reached. “It’s striking how close the Sudanese and South Sudanese feel to Britain in comparison with how we feel about them. In some ways we have forgotten. It comes, for example, from people whose fathers and grandfathers worked in the colonial service. You still hear a lot about it.”

Amid pessimism elsewhere about the hopes for peace, British officials are still understood to be expecting to meet Machar, who is now residing in South Africa in circumstances that are unclear. Reports suggesting he is under house arrest are rejected by some sources.

Others suggest Britain is viewed rather differently in South Sudan, and are concerned about UK troops being put in harm’s way in a country where the government has an increasingly belligerent stance towards UN forces.

“There is gratitude for the UK’s role in the peace agreement with Sudan, but among others there is a residual sense of betrayal dating back to the 1950s. There is a sense that Britain sold the South Sudanese down the river to the Arabs,” said Mawan Muortat, a London-based South Sudanese analyst.

That said, he added that British troops – mainly engineers who will reinforce fortifications around camps used to protect displaced people – were likely to be welcomed and seen by some as a more professional force in comparison with other UN forces, which are seen as impotent or untrustworthy.

The troops’ arrival comes as atrocities continue, including last Monday’s killing of 16 civilians in attacks blamed on a government-aligned militia. Such brutality is a far cry from the hopes invested in South Sudan’s independence in 2011, and the dreams fostered decades earlier by the UK-based students who absorbed ideas of black power and took inspiration from other liberation movements.

More recent British links may yet turn out to be pivotal, ranging from the role of Anglican church leaders to significant state expenditure. While the most high-profile example of the latter is an aid package of £100m distributed via the UN, NGOs and others, other smaller amounts of funding could be important.

South Sudan has benefited from a little-known £1bn UK government fund designed to build stability overseas – the Conflict, Stability and Security Fund (CSSF) – details of which have been seen by the Observer. More than £500,000 was spent by the Foreign Office on a “Preventing Sexual Violence in Conflict Initiative” pilot, while more than £2m went to conflict prevention programmes that also involved Sudan. The Ministry of Defence was involved in smaller (£200,000) spending in Sudan listed as “South Sudan – Defence Engagement”.

British officials appear eager at least to emphasise that the UK has a stake when it comes to the search for peace, and to explain why troops are being sent into a war zone where the UK’s interests appear hard to determine.

“Having troops on the ground is as much about our recommitting to UN peacekeeping as it is to specifically South Sudan,” said one.

“But obviously East Africa is also an important part of the globe and we would want to ensure it remains stable or indeed improves. We don’t have huge economic interests there right now, although South Sudan has potential. Somalia is next door as well, so it’s really beholden on us if we want to see a stable east Africa that we resolve South Sudan’s conflict.”