| Iron Mountain (UK) Data Centre Limited | Transport & logistics | 2017 |
| Iron Mountain (UK) EES Holdings Limited | Financial services | 2014 |
| Iron Mountain (UK) PLC | Transport & logistics | 1980 |
| Iron Mountain (UK) Services Limited | Transport & logistics | 2012 |
| Iron Mountain Dims Limited | Financial services | 1992 |
| Iron Mountain Europe (Group) Limited | Financial services | 2006 |
| Iron Mountain Europe Limited | Professional & scientific | 1988 |
| Iron Mountain Holdings (Europe) Limited | Professional & scientific | 1999 |
| Iron Mountain International (Holdings) Limited | Information & communication | 2012 |
| Iron Mountain MDM Limited | Professional & scientific | 1979 |
| Iron Mountain UK Services (Holdings) Limited | Professional & scientific | 2012 |
| Isabeth Properties Ltd | Real estate | 2021 |
| Ishiji Ltd | Real estate | 2019 |
| Ishwinder Suri Ltd | Other services | 2014 |
| ISI Global Ltd | Professional & scientific | 2009 |
| Island View Ltd | Other services | 2020 |
| Istanbul Barber 23 Ltd | Other services | 2023 |
| It's a Retro Thing Ltd | Other services | 2024 |
| Italian Manufacturing Ltd | Manufacturing | 2012 |
| Itech Management (London) Ltd | Information & communication | 2008 |
| Itty London Ltd | Wholesale & retail | 2014 |
| Ivatt Investments Ltd | Real estate | 2024 |
| IX Bees Properties Limited | Real estate | 2014 |
| J a Manufacturing Ltd | Manufacturing | 2000 |
| J and J Training Limited | Professional & scientific | 2017 |
| J Brinkley & Co Limited | Hospitality | 2007 |
| J C W International Limited | Administrative & support | 2007 |
| J Deacon Building and Roofing Ltd | Construction | 2013 |
| J Deane Medical Limited | Other services | 2012 |
| J Flint Consultancy Limited | Administrative & support | 2011 |
| J H Clarke & Co Limited | | 1947 |
| J J Quinn & SON Ltd | Construction | 2010 |
| J Kirk Medical Ltd | Health & social care | 2025 |
| J M Fussey Ltd | Health & social care | 2022 |
| J M Wildbore Ltd | Wholesale & retail | 2023 |
| J P Skues Transport Limited | Transport & logistics | 2004 |
| J R Dexter Limited | Wholesale & retail | 2004 |
| J Smith Construction Services Limited | Construction | 2014 |
| J T C Wright & Sons Limited | Transport & logistics | 2018 |
| J Williamson Developments Ltd | Real estate | 2024 |
| J. Coates (Garage Services) Limited | Wholesale & retail | 1988 |
| J. Coates Contract Hire Limited | Administrative & support | 2003 |
| J.E.T. Industrial Supplies Ltd | Wholesale & retail | 2021 |
| J.O.C Couriers Limited | Transport & logistics | 2025 |
| J.S.H. Ltd. | Transport & logistics | 1994 |
| J3M Construction Training Limited | Administrative & support | 2016 |
| J3M Tunnelling(Midlands) Limited | Professional & scientific | 2022 |
| J5 Consultancy Limited | Professional & scientific | 2021 |
| Jackson Surtees Ltd | Professional & scientific | 2011 |
| Jacob Jurgensen (UK) Limited | Other services | 1998 |
| Jacob Sykes Limited | Real estate | 2006 |
| Jactra Limited | Construction | 2009 |
| Jade Monkey Audio Ltd | Information & communication | 2026 |
| JAG Consultancy Limited | Professional & scientific | 2020 |
| Jahlis Limited | Information & communication | 2001 |
| James O Burton Ltd | Health & social care | 2019 |
| James Sadler Limited | Other services | 2011 |
| Jamie Gregory Ltd | Professional & scientific | 2017 |
| JAN Kovac Limited | Other services | 2005 |
| Jasmine House B&B Limited | Hospitality | 2015 |
| Jason Higgins Tree Services Limited | Administrative & support | 2008 |
| JAW Maintenance & Systems Limited | Construction | 2015 |
| Jawbilly Enterprises Limited | Administrative & support | 2012 |
| Jaycee Automotive Ltd | Wholesale & retail | 2017 |
| Jayden Investments Limited | Extraterritorial | 2021 |
| Jayden Real Estate Limited | Real estate | 2023 |
| Jayne Elliott Hair Company Limited | Other services | 2008 |
| JC Build Limited | Construction | 2011 |
| Jcda Limited | Other services | 2017 |
| JCH Business Solutions Limited | Professional & scientific | 2015 |
| JCT20 Ltd | Wholesale & retail | 2024 |
| Jd.Com International UK Ltd | Wholesale & retail | 2018 |
| JDA 100 Ltd | Professional & scientific | 2010 |
| Jdmoore Limited | Construction | 2022 |
| JDP Property Developments Limited | Construction | 2017 |
| JDS64 Ltd | Administrative & support | 2023 |
| Jeduri Property Solutions Ltd | Professional & scientific | 2015 |
| Jeep Spares 4 U Limited | Wholesale & retail | 2007 |
| Jeff Adkin Construction Limited | Construction | 2001 |
| Jeff Barnwell Ltd | Professional & scientific | 2015 |
| JEM Health Ltd | Health & social care | 2025 |
| Jenkinson & Associates Limited | Professional & scientific | 2006 |
| JET Kings Leicester Ltd | Administrative & support | 2023 |
| Jetstream Motorsport (Bruntingthorpe) Ltd. | Wholesale & retail | 2008 |
| JG Fabrications (Holdings) Limited | Financial services | 2018 |
| Jigsaw Puzzles Ltd | Professional & scientific | 2008 |
| JJG Roofing Ltd | Construction | 2025 |
| Jkarya Investments Ltd | Real estate | 2024 |
| Jklm Management Ltd | Administrative & support | 2012 |
| JKR Engineering and Construction Services Limited | Construction | 2021 |
| JL Logistics & Install Ltd | Transport & logistics | 2025 |
| JLR Classics Limited | Wholesale & retail | 2019 |
| JMH Fabrications Limited | Manufacturing | 2004 |
| JMK Oak Framing Ltd | Professional & scientific | 2025 |
| JMR Autos Ltd | Wholesale & retail | 2023 |
| John Clarke (Natural Stonework) Limited | Construction | 2003 |
| John Durrance Limited | Hospitality | 2019 |
| John Jenkins Photography Limited | Other services | 2013 |
| John Lester Partnership Limited | Professional & scientific | 1996 |
| John Shopland Limited | Construction | 2016 |