| J&A Welding and Fabricating Ltd | Manufacturing | 2026 |
| J&I Consulting Limited | Professional & scientific | 2019 |
| J&S Design Ltd | Administrative & support | 2017 |
| J&T (UK) Retail Limited | Wholesale & retail | 2018 |
| J. Coad Limited | Health & social care | 2004 |
| J. DE Walleg Limited | Manufacturing | 1976 |
| J. Lloyd and Sons (Farms) Limited | Agriculture & fishing | 1977 |
| J.Bras Plumbing & Heating Ltd | Construction | 2019 |
| J.D Carpentry & Building Services Ltd | Construction | 2026 |
| J.Designs Carpentry & Flooring Ltd | Construction | 2024 |
| J.H.Midwinter Limited | Real estate | 1956 |
| J.K. Installations Limited | Construction | 1998 |
| J.M.B Precision Ltd | Professional & scientific | 2021 |
| JA Real Estate Ltd | Real estate | 2024 |
| JA Taste 4 Real Ltd | Other services | 2025 |
| JAC Baker Ltd | Construction | 2020 |
| JAC Flight Services Ltd | Professional & scientific | 2026 |
| Jach Creatives Ltd | Administrative & support | 2018 |
| Jack Hammer Property Ltd | Households | 2025 |
| Jack the Gripper Ltd | Information & communication | 2015 |
| Jack's Garden & Landscape Maintenance Limited | Administrative & support | 2019 |
| Jackanoryjones Limited | Manufacturing | 2016 |
| Jacob Edmonds Ltd | Construction | 2023 |
| Jacobs Kennedy & Associates Limited | Professional & scientific | 2010 |
| Jadehouse Aylesbury Ltd | Hospitality | 2025 |
| Jaffna Infotech Limited | Information & communication | 2018 |
| Jagge Limited | Professional & scientific | 2015 |
| Jaghomz Limited | Real estate | 2024 |
| Jaibee Ltd | Health & social care | 2025 |
| Jak's ABC Limited | Other services | 2023 |
| Jakana Trading Co Limited | Wholesale & retail | 2002 |
| Jake Collinge Planning Consultancy Limited | Professional & scientific | 2008 |
| Jally Ltd | Information & communication | 2024 |
| JAM & Jelly Productions Limited | Information & communication | 2014 |
| Jambr Holdings Ltd | Construction | 2020 |
| James Bush Surveying Ltd | Professional & scientific | 2025 |
| James Lawson Holdings Limited | Professional & scientific | 1964 |
| James Pettit & Company Limited | Professional & scientific | 2001 |
| James Smith Landscape & Garden Design Ltd | Professional & scientific | 2021 |
| Jamhawks Trading Ltd | Wholesale & retail | 2019 |
| Jamline Ltd | Wholesale & retail | 2024 |
| Jammy Media Limited | Information & communication | 2012 |
| Jammz Ltd | Professional & scientific | 2023 |
| Jamtastic Design Limited | Professional & scientific | 2014 |
| Jamtse Limited | Wholesale & retail | 2007 |
| JAN FOO Limited | Public administration | 2018 |
| Jana Trading Ltd | Wholesale & retail | 2020 |
| Jangid UK Ltd | Other services | 2018 |
| Janshre Ltd | Real estate | 2025 |
| JAS Bhatia UK Ltd | Wholesale & retail | 2021 |
| JAS Information Technology Ltd | Information & communication | 2023 |
| JAS Investments London Ltd | Real estate | 2021 |
| JAS Management Services Ltd | Professional & scientific | 2015 |
| JAS Properties London LLP | | 2026 |
| Jasmine and Luke Limited | Real estate | 2016 |
| Jasmine KC Ltd | Construction | 2022 |
| Jasmine Proprietary Limited | Households | 1989 |
| Jason's Transport Management Services Limited | Transport & logistics | 2023 |
| Jason's Transport Services Limited | Transport & logistics | 2015 |
| JASS26 Ltd | Hospitality | 2026 |
| Jassy Homes Limited | Real estate | 2018 |
| Jatheesan Limited | Construction | 2011 |
| Jatinder Benepal Limited | Health & social care | 2011 |
| JAW Building and Carpentry Services Ltd | Construction | 2025 |
| JAY MCC Limited | Health & social care | 2024 |
| JAY Mills Travel Limited | Transport & logistics | 2023 |
| JAY Newport Accountants Ltd | Professional & scientific | 2013 |
| Jaycee & JAY Ltd | Agriculture & fishing | 2021 |
| Jaykay Associates Limited | Administrative & support | 2010 |
| Jayson Arts Ltd | Wholesale & retail | 2022 |
| JB Sports Associates Limited | Administrative & support | 2018 |
| JC and SD Property Services Ltd | Real estate | 2024 |
| JC Fine Life Products Ltd | Wholesale & retail | 2015 |
| JCA Property Investments Ltd | Real estate | 2025 |
| Jceh Ltd | Wholesale & retail | 2026 |
| JD Exploration Consultants Limited | Professional & scientific | 2023 |
| JDK Holdings Ltd | Wholesale & retail | 2023 |
| JDP Change Leadership Limited | Professional & scientific | 2014 |
| JDR Consulting Limited | Information & communication | 2013 |
| Jemw Ltd | Professional & scientific | 2008 |
| Jenkins Fitness Ltd | Arts & recreation | 2025 |
| Jenn BI Ltd | Information & communication | 2025 |
| Jennie Robbins Limited | Extraterritorial | 2016 |
| Jeran Limited | Transport & logistics | 2018 |
| Jersey Milk Producers Limited | Professional & scientific | 2023 |
| Jesh Ltd | Information & communication | 2014 |
| Jessies Healthcare Limited | Health & social care | 2019 |
| Jessnat Ltd | Wholesale & retail | 2018 |
| Jester Media Limited | Information & communication | 2013 |
| JET Solutions Limited | Construction | 2024 |
| JET Taxis Aylesbury Ltd | Transport & logistics | 2017 |
| Jetfruit Ltd | Hospitality | 2016 |
| JFC Aylesbury Limited | Hospitality | 2026 |
| JFS Group Holdings Limited | Financial services | 2025 |
| JFS Interiors Limited | Construction | 2026 |
| Jfstays Ltd | Hospitality | 2024 |
| JG Building Projects Limited | Construction | 2022 |
| JGI Consulting Ltd | Professional & scientific | 2025 |
| JH Capital | Administrative & support | 2019 |
| JHT Business Consultants Limited | Professional & scientific | 2010 |