Kweku Adoboli considers himself “very British, but more a Yorkshireman”. He has lived in the UK for the last 26 years, paid UK taxes and his favourite desert is apple crumble.

But in the eyes of the law, Adoboli is not British, and his failure to get around to applying for citizenship has left him facing deportation to Ghana, his country of birth which he left as a four-year-old. He could be sent to Africa as soon as Monday, and is spending what could be his last few days in Britain in a notorious Scottish detention centre.

Adoboli, 38, was found guilty in 2012 of being responsible, in the words of the judge, “for the largest trading loss in British banking history”. The former UBS banker lost his Swiss bank $2.3bn (£1.8bn) in “off-book trades” that brought the bank to its knees, leading to 10,000 job cuts and the axing of its chief executive.

Adoboli was sentenced to seven years in jail, of which he served four. Under UK law, foreign nationals sentenced to more than four years in jail are automatically subject to deportation.

From the canteen in the Dungavel immigration removal centre, near Strathaven, Adoboli is appealing to home secretary Sajid Javid and prime minister Theresa May to “show some compassion” and intervene to prevent his “banishment”.

“I did four years in prison for a non-violent act,” he said in an interview with the Guardian from the detention centre. “I’ve served my time, but I’m being given an extra punishment – a banishment – and just because I didn’t get around to becoming a citizen. This punishment is so much worse than being in prison.

“Prison is a finite; you do your time, you hold your head high and you know you will get out. This [being deported] is really inhumane.”

Adoboli believes there are racist overtones to his treatment. “People say it is OK for me to be sent back because it is where I am from,” he said. “But I left when I was four. My life is here, my friends are here. If I were British my punishment would be over and I could get on with life.”

Not becoming a British citizen when he had the opportunity is, he said, the biggest oversight of his life. “Ironically, it was because I was trying to be good at my job that I didn’t go through the citizenship steps,” he said. “I was the public face of UBS’s Delta One desk running a £50bn portfolio. At the drop of a hat I had to get on a plane to Frankfurt, New York or the Eurostar to Paris. I kept my passport in my desk drawer.”

Adoboli, who was earning £350,000 a year in his 30s, said he wouldn’t have been able to do his job if he sent his passport to the Home Office as part of his citizenship application. “I had indefinite leave to remain, which the bank said was basically the same as citizenship,” he said.

He has no fears about life in Ghana, but it’s the prospect of never being able to return to the UK that gives him nightmares. “I won’t see my girlfriend, friends or seven godchildren again.”

Until he was detained by immigration officers this week, he was staying with the parents of two of his godchildren in Livingston. “One of them is five, the other is three, and they don’t know where I am. They think I have gone down to London to earn some pennies, but they have figured out something is wrong. I normally tell them how many ‘sleeps’ I will be away for – but this time I didn’t.”

The son of a Ghanaian UN peacekeeper, Adoboli’s family left Ghana when he was four and he lived in Israel, Syria and Iraq. At 12 he was sent to a Quaker boarding school in West Yorkshire. “My friends at boarding school became my siblings, their parents became my surrogate family.”

From boarding school he went to Nottingham University where he studied computer science and management. “I ended up in banking by mistake,” he said. “I needed to do an internship as part of my course, I wanted to do it at a tech firm but this was 2002 after the dotcom bubble had burst and no one was hiring, but the banks were.”

He rose quickly, and was sent around the world. Soon he was appointed to run the Delta One desk with colleague John Hughes. “It was a $50bn book being run by two kids,” he said.

Adoboli, who says he was working 20 hours a day and hating it, claimed he was preparing to quit in the weeks before the bank discovered his unauthorised trades. He admitted to the trades and set out the details in an email to his bosses, in which he took responsibility for “the shitstorm that will now ensue”. He was arrested at 3.35 the next morning at his £1,000-a-week flat near work.

He can understand that the public want to see bad bankers punished, but he said regular bankers like him were just trying to please their bosses and said the system needs to change. “That’s what I have been trying to do,” he said. “I am volunteering, teaching ethics to finance kids and speaking about the need for responsible leadership and accountability. We’re trying really hard to create cultural change and rebuild the social construct between the City and the public.”

Adoboli said his work has included training courses for the Ministry of Defence, the Ministry of Justice, the Home Office and Barclays. On one three-day officer development course for the UK Special Forces he and Tony Blair were the keynote speakers.

“I am trying to teach others, including the Home Office, how to learn from my mistakes,” he said. “But they want me out of the country.”

• This article was amended on 10 September 2018 to correct misspellings of Kweku Adoboli’s last name from Adobli.