The long-awaited release from detention in Iran of two UK-Iranian dual nationals, Nazanin Zaghari-Ratcliffe and Anoosheh Ashoori, signals, on the face of it, a constructive turn in UK-Iranian relations. It was brought about by the tireless behind-the-scenes work of advocates and officials , from progress in the Vienna nuclear talks to support from the Omani government and an international public pressure campaign. Yet, despite this step forward, more challenging issues between Tehran and Whitehall are looming. Not least is the fear that by paying its 40-year-old £400m debt to Iran, the British government risks vindicating Iran’s use of hostage-taking. It is for this reason that since Zaghari-Ratcliffe’s detention, the British government has been rightfully reluctant to link the prospect of her release to the debt. Indeed, in 2016, then US president Barack Obama was heavily criticised for a similar repayment of $400m to Tehran after the release of four US-Iranian dual nationals. To comply with sanctions and counter-terrorism and money laundering regulations – and to prevent a repeat of the Obama optics – the Foreign Office has announced that the UK debt money has been ringfenced, and will be used for humanitarian purposes only. However, hostage-taking remains an ongoing issue. The FCDO currently advises British-Iranian dual nationals against travelling to Iran, and two are still trapped there: Mehran Raoof has been sentenced to 10 years in prison on national security charges, and environmental activist and campaigner Morad Tahbaz – who also holds US citizenship – was granted furlough from prison but cannot leave Iran. Tahbaz is reportedly being treated as a US detainee by the Iranian government. His case, alongside those of Siamak and Baquer Namazi and Emad Sharghi remain in limbo, and are connected to the outcome of the continuing nuclear negotiations between the US and Iran. The Biden administration returned to the negotiating table in April 2021, leading to the slight progress that played a part in the release of Ashoori and Zaghari-Ratcliffe. These nuclear talks hold the key to Iran’s future on the world stage, and to whether its nuclear programme continues to ramp up. The UK’s debt repayment had been further delayed by the Trump administration’s 2018 withdrawal from the Iran nuclear agreement (the JCPOA), along with the maximum-pressure sanctions it imposed on Iran. Yet Despite Trump’s withdrawal from the deal and Tehran’s subsequent nuclear accelerations – which have reduced Iran’s nuclear “breakout time” from one year to a number of months – the UK, alongside France and Germany, worked to hold what was an effective multilateral deal together through the trying Trump years. After 11 months of negotiations, the end of the nuclear talks is indeed within sight, but last-minute issues require resolution. It is expected that most of the Trump-era sanctions imposed on Iran will be removed in exchange for Tehran’s return to the limits agreed in the original 2015 nuclear deal. Most importantly, Iran’s programme would again be subject to the oversight and monitoring of the International Atomic Energy Agency (IAEA). Yet last week, Russia, which has thus far been a constructive party in the negotiations, attempted to sabotage their progress by demanding written guarantees to protect its participation in the deal from Ukraine-related sanctions. This was resolved when the Biden administration provided Moscow with the necessary assurances that sanctions over Ukraine would not hamper their cooperation and support of the agreement. What remains now is for the negotiators to finalise the last remaining sticking points and return to Vienna. In the final stretch, Tehran has final demands: it is seeking more precise economic guarantees to protect the JCPOA should a future US president again withdraw from the deal, along with the removal of the Islamic Revolutionary Guard Corps from the US list of terrorist groups. These are not straightforward concessions for the US. Republican opposition to the JCPOA and criticism of the Biden administration’s negotiations with Tehran in a critical election year continue to tie the president’s hands in making bold concessions – and the IRGC’s flagrant missile attack last weekend in Erbil, Iraq will be hard for the US, UK and Europe to overlook. Arriving at a deal will require all sides to weigh the progress that’s been made against the spiralling risks of a collapse. A no-deal scenario would no doubt see Tehran accelerate its nuclear programme further, and would increase regional tensions, particularly with Israel. Gulf Arab states are also legitimately concerned that Iran’s regional activities remain unchecked. The UAE and Saudi Arabia in particular have been made vulnerable to missile strikes from the Yemen-based, Iran-backed Houthi group. Regardless of the deal or no-deal scenarios, fragile regional security dynamics require attention and support from the international community. The release of Zaghari-Ratcliff and Ashoori should be seen as an important turning point in the UK’s dealings with Iran – but we are still far from resolution. Sanam Vakil is a senior research fellow in the Middle East and North Africa programme at Chatham House