| J a Whitney Building Contractors Limited | Administrative & support | 2018 |
| J Benfield Media Ltd | Information & communication | 2020 |
| J Berry ( Building ) Ltd | Construction | 2011 |
| J D M Plumbing Services Limited | Construction | 2025 |
| J Denman Enforcement Limited | Administrative & support | 2017 |
| J E Upperlimb Ltd | Health & social care | 2021 |
| J F Webb Electrical Limited | Construction | 2012 |
| J K Fitness Limited | Professional & scientific | 2004 |
| J M Creative Landscapes Limited | Other services | 2015 |
| J Muir Bookbinders Limited | Manufacturing | 2007 |
| J P Frain Carpentry Limited | Construction | 2008 |
| J T Fire Risk Management Limited | Public administration | 2016 |
| J Wells Developments Ltd | Construction | 2018 |
| J&A Duct Ltd | Construction | 2020 |
| J&E (International) Consulting Co., Ltd | Professional & scientific | 2025 |
| J&I Plumbing and Heating Limited | Construction | 2023 |
| J&J Automotive Ltd | Wholesale & retail | 2010 |
| J&M Re-Planning Solutions Limited | Transport & logistics | 2019 |
| J.A Wickes Taxis Limited | Transport & logistics | 2019 |
| J.H. Bloomfield & SON Limited | Real estate | 1946 |
| J.Richards Ground Maintenance Limited | Construction | 2025 |
| J1 Plastering & Rendering Ltd | Construction | 2023 |
| J2 Consultancy Ltd | Professional & scientific | 2025 |
| JAB Management Group Ltd | Professional & scientific | 2016 |
| Jace Group UK Limited | Construction | 2018 |
| Jack Smith Design Ltd | Administrative & support | 2023 |
| Jacob Samuel Ltd | Financial services | 2009 |
| Jacob Shooter Lighting Design Ltd | Administrative & support | 2018 |
| Jacobs Safety Services Ltd | Extraterritorial | 2005 |
| Jade Peirson Ltd | Other services | 2019 |
| Jadrr 25 Ltd | Administrative & support | 2025 |
| Jakco Consulting Limited | Real estate | 2010 |
| JAM Brighton Ltd | Arts & recreation | 2021 |
| James & Sons Plumbing and Heating Limited | Construction | 2009 |
| James Baker Consultancy Ltd | Professional & scientific | 2021 |
| James Bisby Limited | Administrative & support | 2021 |
| James Garnham Design Ltd | Professional & scientific | 2020 |
| James Gorey Ltd | Real estate | 2019 |
| James Hardinge Ltd | Administrative & support | 2011 |
| James LEE Flooring Ltd | Construction | 2014 |
| James LEE Ltd | Information & communication | 2024 |
| James Reeds Limited | Administrative & support | 2018 |
| James Software Solutions Limited | Information & communication | 2014 |
| James Wright Heating Services Ltd | Construction | 2022 |
| Jamesy Ltd | Professional & scientific | 2019 |
| Jamie's Diner Limited | Hospitality | 2013 |
| Jamie's Events Ltd | Administrative & support | 2023 |
| Jamie’S Services Limited | Transport & logistics | 2024 |
| Jamjar (UK) Limited | Administrative & support | 2012 |
| Janeen London Ltd | Wholesale & retail | 2023 |
| Jansi Fashion Ltd | Wholesale & retail | 2012 |
| Janson Court (Sidcup) Management Limited | Households | 1979 |
| JAP Electrical Limited | Construction | 2020 |
| JAS Electrical and Building Ltd | Administrative & support | 2022 |
| JAS Transport Ltd | Transport & logistics | 2026 |
| Jasella Limited | Real estate | 2021 |
| Jason Dolby Limited | Administrative & support | 2015 |
| Jason Hastings Limited | Administrative & support | 2023 |
| Jason West Limited | Administrative & support | 2005 |
| Jayne's Homes Limited | Health & social care | 2021 |
| Jazzy JAY Limited | Information & communication | 2017 |
| JB Chauffeuring Services Ltd | Transport & logistics | 2023 |
| JB Commissioning Services Limited | Professional & scientific | 2023 |
| JBC Grand Homes Ltd | Construction | 2007 |
| JBH Real Estates Ltd | Real estate | 2025 |
| Jboyz Limited | Wholesale & retail | 2021 |
| JBW Electrical Services Limited | Construction | 2024 |
| JC O'Sullivan Property Limited | Real estate | 2018 |
| JC Three Cranes Ltd | Hospitality | 2024 |
| Jcos Consultancy Limited | Professional & scientific | 2025 |
| Jcso Investments Limited | Administrative & support | 2017 |
| JD Design and Management Ltd | Professional & scientific | 2023 |
| JD Kent Electrical Ltd | Administrative & support | 2022 |
| JDF Electrical Services Ltd | Construction | 2018 |
| JDG Restoration and Repointing Ltd | Construction | 2025 |
| JDM Building Services Ltd | Construction | 2015 |
| JDR Commissioning Ltd | Professional & scientific | 2022 |
| Jdstr Ltd | Real estate | 2020 |
| JE Oseman Building Services Limited | Construction | 2010 |
| Jehl Electrics Limited | Construction | 2019 |
| JEM Clothing Limited | Wholesale & retail | 2014 |
| Jemasumil Ltd. | Wholesale & retail | 2021 |
| Jems Food Ltd | Hospitality | 2024 |
| Jenner Communications Ltd | Administrative & support | 2020 |
| Jennys Ltd | Arts & recreation | 2011 |
| Jensen Motor Group Limited | Manufacturing | 2022 |
| Jentap Holdings Limited | Real estate | 2023 |
| Jeron Shop Management Ltd | Professional & scientific | 2026 |
| JES Electrical Services Ltd | Energy | 2017 |
| JES Estate Agents Limited | Administrative & support | 2022 |
| Jessica Chamberlain Limited | Professional & scientific | 2012 |
| Jesworld Ltd | Extraterritorial | 2003 |
| JET Construction Limited | Construction | 2012 |
| JET Renovations Ltd | Real estate | 2018 |
| Jeunepine Limited | Real estate | 2016 |
| Jewelato Ltd | Manufacturing | 2018 |
| Jewellery Repair Centre Limited | Administrative & support | 2004 |
| Jewellery Workshop Design Limited | Professional & scientific | 2012 |
| Jewellery Workshop Repairs Limited | Other services | 2012 |
| Jewellery Workshop Retail Limited | Wholesale & retail | 2012 |