The UK does not extradite prisoners to countries where they will not receive a fair trial. Assange will not receive a fair trial in the United States.
Under the ECHR, any European country facing an extradition request is obliged to consider the individual’s ability to receive a fair trial in the country. Assange's case is inherently political. This could result in an unfair trial, in which case, the UK should not extradite Assange.
Given the nature of his crimes, Assange would be unlikely to receive a fair trial in the US legal system. The US government appoints the country’s judges, and the government was on the receiving end of Assange’s leaks. It would be impossible for him to receive fair and impartial treatment in the US judiciary. A US judge would be able to seal evidence and restrict the defence team’s access to government documents. It could even meet in private with the prosecution team, as occurred in the high-profile case of whistleblower John Kiriakou when he was accused of espionage after revealing details of the US government’s illegal use of torture. The Eastern District of Virginia is known as the ‘Espionage court’ and illustrates how weighted the US legal system is against whistleblowers and those accused of exposing national security secrets. No national security defendant has ever won a case there.
The right to a fair trial in all civil and criminal proceedings that take place on US soil is enshrined in the US Fourteenth Amendment in the Constitution. The US Justice System also meets the international requirements for an independent and impartial judiciary as laid out in article 14 of the OHCHR. The UK, therefore, would have no legal grounds to reject the United States' extradition request over concerns that Assange would not receive a fair trial in the US legal system.
[P1] The UK is obliged to consider whether the defendant would receive a fair trial before approving their extradition. [P2] Assange would not receive a fair trial in the US. [P3] Therefore, the UK should not approve the extradition request.
[Rejecting P2] Assange would receive a fair trial.