https://www.state.gov/documents/organization/187784.pdf - Treaty Between the
UNITED STATES OF AMERICA
and the UNITED KINGDOM OF
GREAT BRITAIN AND
NORTHERN IRELAND
Signed at Washington March 31, 2003
UK: The US/UK 2003 Extradition Treaty - An American Perspective?
https://en.wikipedia.org/wiki/UK%E2%80%93US_extradition_treaty_of_2003
https://en.wikipedia.org/wiki/Extradition_Act_2003
The treaty has been claimed to be one-sided[3] because it allows the US to extradite UK citizens and others for offences committed against US law, even though the alleged offence may have been committed in the UK by a person living and working in the UK (see for example the NatWest Three), and there being no reciprocal right; and issues about the level of proof required being less to extradite from the UK to the US rather than vice versa.
Among other provisions Part 2 of the Act: Extradition to category 2 territories (non-European Arrest warrant territories) removed the requirement on the USA to provide prima facie evidence in extraditions from the UK, requiring instead only reasonable suspicion. This was necessary to redress the previous imbalance against the USA under the 1870 Act, as the UK did not have to provide the more onerous prima facie evidence to extradite from the USA. The requirement for the UK is to show probable cause. However, an independent legal review carried out by Sir Scott Baker found that “there is no significant difference between the probable cause test and the reasonable suspicion test. There is no practical difference between the information submitted to and from the United States.
https://www.bbc.co.uk/news/uk-politics-16041824
US-UK extradition: The law explained
Extradition from the UK to US
The process begins when a US prosecutor obtains an arrest warrant with the approval of the domestic courts.
In England and Wales, the Crown Prosecution Service acts on behalf of the US in taking the case through the courts.
Once the police have arrested the suspect, the courts must decide whether or not the US has met the tests required for an extradition to take place. This is a fairly technical hearing in which the lawfulness of the request is examined.
The US does not need to provide all the evidence in a case, as if the allegations were being tested in a full trial by British judges.
What this comes down to is proving to the courts that there is reasonable suspicion - essentially the hurdle police need to jump to justify an arrest - but obviously short of what is necessary to get a conviction.