| J&K Brickwork Ltd | Manufacturing | 2025 |
| J-Clean Services Limited | Administrative & support | 2003 |
| J-Mach Limited | Information & communication | 2013 |
| J. E. Buckle (Holdings) Limited | Financial services | 2012 |
| J.Robarts & SON Limited | Wholesale & retail | 1937 |
| J.Speed Limited | Information & communication | 2010 |
| J.W.Franklin Limited | Agriculture & fishing | 1964 |
| JA Goldwin Investments Limited | Administrative & support | 2021 |
| JA Investing Limited | Real estate | 2023 |
| Jack Johnstone Photography Ltd | Professional & scientific | 2013 |
| Jacks Hill Cafe Limited | Other services | 1960 |
| Jacks Hill Caravans Limited | Hospitality | 1960 |
| Jade Pybus Studio Ltd | Arts & recreation | 2019 |
| JAG Carpentry & Joinery Limited | Construction | 2018 |
| Jaguar Telecom Limited | Information & communication | 2007 |
| Jaiambe Ltd | Wholesale & retail | 2025 |
| Jaine Whitfield Property Services Ltd | Real estate | 2020 |
| Jakk Holdings Limited | Financial services | 2019 |
| Jakopo Limited | Health & social care | 2026 |
| Jaksn Properties Ltd | Real estate | 2025 |
| JAM 90 Ltd | Real estate | 2021 |
| JAM Digitek Ltd | Information & communication | 2024 |
| James Carlon Estates Ltd | Real estate | 2024 |
| James Finn Holdings Limited | Financial services | 2025 |
| James Heritage Limited | Health & social care | 2024 |
| James Howe Landscapes Limited | Administrative & support | 2017 |
| James Potter Ltd | Arts & recreation | 2013 |
| James RAY Heating Limited | Construction | 2010 |
| James Wealth Management Ltd | Financial services | 2010 |
| James Willis Art Limited | Arts & recreation | 2014 |
| Jamie Barton ISW Ltd | Health & social care | 2017 |
| Jamie Hewitt Cricket Academy Limited | Arts & recreation | 2008 |
| Jamie Hewitt Cricket Ltd | Education | 2021 |
| Jammer Ltd | Professional & scientific | 2020 |
| Jaqs Solutions Limited | Construction | 2023 |
| Jaugust Visual Limited | Professional & scientific | 2017 |
| Javelin Technologies Ltd | Administrative & support | 2023 |
| JAY House Commercial Ltd | Real estate | 2022 |
| JAY House Property Limited | Real estate | 2017 |
| JAY Smith Media Ltd | Information & communication | 2025 |
| Jaylec Electrical Contractors Limited | Construction | 1997 |
| Jaysys Limited | Professional & scientific | 2002 |
| JB Advisory Limited | Professional & scientific | 2022 |
| JB Carpentry Limited | Construction | 2005 |
| JB Surveyors Limited | Extraterritorial | 2016 |
| Jbromero Ltd | Transport & logistics | 2024 |
| JC Accounts Limited | Professional & scientific | 2008 |
| JC Illustration Ltd | Arts & recreation | 2014 |
| JCE Consulting Limited | Other services | 2018 |
| JCJ Classics Ltd | Wholesale & retail | 2019 |
| JCJ Holdings Ltd | Manufacturing | 2019 |
| JCJ Property and Leisure Ltd | Real estate | 2019 |
| JCL7 Management Limited | Financial services | 2026 |
| JD & CJ COX Properties Limited | Real estate | 2017 |
| Jdalv Holdings LLP | | 2021 |
| Jdam Group Ltd | Transport & logistics | 2023 |
| JDB Design Limited | Professional & scientific | 2026 |
| JDC Property Limited | Real estate | 2013 |
| Jddw Property Group Limited | Real estate | 2026 |
| JDL Asset Holdings Limited | Financial services | 2020 |
| JDL Construction (Herts) Limited | Extraterritorial | 2012 |
| JDL Construction Group Limited | Extraterritorial | 2025 |
| JDL Construction Limited | Construction | 2010 |
| JDM Consultancy Services Limited | Professional & scientific | 2021 |
| JDS Plant Hire Limited | Administrative & support | 2011 |
| JDS Property Maintenance Services Limited | Construction | 2016 |
| Jdssproperty Ltd | Real estate | 2022 |
| JE Development Enterprise Limited | Real estate | 2021 |
| JEE Properties Ltd | Real estate | 2018 |
| Jeeves Care Group Limited | Health & social care | 2020 |
| Jeeves Holdings (Hertfordshire) Limited | Financial services | 2025 |
| Jeeves Holdings Limited | Financial services | 2018 |
| Jeeves Properties Ltd | Real estate | 2017 |
| Jeffrey and White Management Ltd | Other services | 2007 |
| JEH Consultancy Ltd | Professional & scientific | 2023 |
| Jelprint Limited | Manufacturing | 2011 |
| JEM Initiative Ltd. | Professional & scientific | 2014 |
| Jemshine Investments Limited | Real estate | 1996 |
| Jenni Bello Designs Ltd | Professional & scientific | 2023 |
| Jennie-Wrens Lifestyles Limited | Professional & scientific | 2019 |
| Jennifer Edmonds Property Consulting Ltd | Real estate | 2025 |
| Jepps Sporting Limited | Arts & recreation | 2023 |
| Jessica Pressland Ltd | Education | 2012 |
| Jetforcesolutions Ltd | Administrative & support | 2025 |
| Jetta Services Ltd | Administrative & support | 2025 |
| JG Auto Services UK Ltd | Wholesale & retail | 2025 |
| JGB Accountants and Business Advisors Ltd | Professional & scientific | 2020 |
| JGP London Limited | Arts & recreation | 2016 |
| JHC Laundry Equipment Limited | Wholesale & retail | 1991 |
| JHK Business Valuations & Sales Limited | Administrative & support | 2025 |
| JHK Limited | Financial services | 2022 |
| Jill Coombs Interiors Ltd | Wholesale & retail | 2018 |
| Jivebird Limited | Information & communication | 2013 |
| JJ Diagnostics Ltd | Health & social care | 2024 |
| JJ Investments Ltd | Real estate | 2011 |
| JJH Consultancy Ltd | Education | 2025 |
| JK UK Properties Limited | Real estate | 2022 |
| JKJ Trading Ltd | Wholesale & retail | 2019 |
| JKP Financial Ltd | Financial services | 2022 |
| JL & WE Holdings Ltd | Financial services | 2024 |