Policing and Crime Act 2017, Part 6 Firearms, Section 125
http://www.legislation.gov.uk/ukpga/2017/3/section/125/enacted
[Thank you to @Sacarathefor proving the link]
Here is a link to the Home Office circular 031 / 2007 The Violent Crime Reduction Act 2006 (Commencement No 3) Order 2007: firearms measures
https://www.gov.uk/government/publications/the-violent-crime-reduction-act-2006-commencement-no-3-order-2007-firearms-measures