Christopher Steele is using the UK-US extradition laws because the UK will not extradite if it's a death penalty case.
[1] The UK-US extradition treaty was signed on 31 March 2003 and came into force in April 2007, following the US Senate's ratification of the Treaty. The UK's arrangements for extradition to the US set out in the Treaty came into force in advance of this date as they were included within the Extradition Act 2003. As with the European Arrest Warrant, a person can be extradited under the Treaty for any offence with a minimum sentence of one year.
The Treaty provides for the UK to refuse extradition where the offence for which extradition is sought is punishable by the death penalty except where "the Requesting State provides an assurance that the death penalty will not be imposed or, if imposed, will not be carried out."
https://publications.parliament.uk/pa/jt201012/jtselect/jtrights/156/15608.htm
Controversy surrounds the UK–US extradition treaty of 2003, which was implemented by the UK in the Extradition Act 2003 and came into force in April 2007 following its ratification by the US Senate in 2006.[1][2] Christopher Steele, The Storm is now upon you.
https://en.wikipedia.org/wiki/UK%E2%80%93US_extradition_treaty_of_2003#cite_note-1