- Video report by ITV News Security Editor Rohit Kachroo
A former British inmate at Guantanamo Bay was reportedly given £1m in compensation for his detention.
Abu Zakariya al-Britani reportedly died fighting for so-called Islamic State in Iraq.
IS said al-Britani blew himself up on Monday as Iraqi forces approached Mosul.
The 50-year-old, a Muslim convert born Ronald Fiddler and also known as Jamal al Harith or Jamal Udeen, was suspected of terrorism by the Americans but freed from the US detention centre in 2004 after lobbying by the British government.
When he returned from Guantanamo in 2004, he spoke to ITV News and described the "harsh" conditions inside the centre.
The subject of Al-Britani's substantial compensation has led to a row between former Prime Minister Tony Blair and tabloid newspaper 'The Daily Mail'.
In Wednesday's paper the Mail questioned why the then Labour government paid al-Britani compensation, but posting a response on Twitter, Mr Blair said the payoff was agreed in 2010 by the Conservative-led coalition government.
Mr Blair also said that the Mail was at the forefront of a campaign to get al-Britani and other British captives in Guatanamo Bay released and returned to the UK.
Al-Britani turned to Islam in the 1990s and travelled to the Pakistani city of Quetta in 2001 for what he claimed was a religious holiday.
He has insisted he tried to enter Iran when the US invaded neighbouring Afghanistan, but was captured and imprisoned by the Taliban on suspicion of being a UK spy.
Al-Britani's brother, Leon Jameson, 53, identified him as the man equipped with explosives in an IS propaganda video.
Mr Jameson told The Times his brother had "wasted his life" and added: "It is him, I can tell by his smile."
Around 850 individuals of national security concern have travelled to join the conflict, according to figures published by the Government last year.
Of those, just under half have returned to the UK and approximately 15% are dead.
- Analysis by ITV News Security Editor Rohit Kachroo