| J K International Trading Ltd | Wholesale & retail | 2017 |
| J Mann and Co Ltd | Professional & scientific | 2012 |
| J Schmid Landscapes Limited | Administrative & support | 2019 |
| J T Gilbert Construction Limited | Construction | 2015 |
| J Thoria Limited | Health & social care | 2021 |
| J Villa Cleaning Limited | Administrative & support | 1998 |
| JA Construction Services Limited | Administrative & support | 2007 |
| Jaan Corp Ltd | Information & communication | 2013 |
| Jack Davies Limited | Information & communication | 2023 |
| Jaderberg Consulting Ltd | Professional & scientific | 2022 |
| JAI Shri RAM Ltd | Professional & scientific | 2011 |
| Jaisswal Limited | Real estate | 2025 |
| Jalan Medical Consultants Limited | Health & social care | 2010 |
| Jamie Rous Technical Services Limited | Professional & scientific | 2007 |
| Jamielle Ltd | Real estate | 2023 |
| Jamm Recruitment Ltd | Administrative & support | 2016 |
| Jana Consulting Ltd | Information & communication | 2017 |
| Janus Properties Limited | Real estate | 2016 |
| Jari Investments Ltd | Real estate | 2021 |
| Jarrahwood Limited | Construction | 2011 |
| Jasmin RTM Company Limited | Extraterritorial | 2026 |
| Javar Ltd | Real estate | 2011 |
| Jaxel Investments Ltd | Financial services | 2021 |
| Jaxminster Limited | Professional & scientific | 1982 |
| JAY Bytes Solutions Ltd | Information & communication | 2017 |
| Jayshree Krishna Ltd | Real estate | 2024 |
| JCM Radiology Reporting Services Ltd | Health & social care | 2020 |
| JCR Business Services Limited | Professional & scientific | 2006 |
| JCR Property Management Limited | Real estate | 2016 |
| JCT International Limited | Information & communication | 2025 |
| Jeeks Ltd | Professional & scientific | 2017 |
| Jeff's Clothes Show Limited | Real estate | 2000 |
| Jefferson Gore Ltd | Administrative & support | 2015 |
| Jefferson Sales Consultancy Ltd | Professional & scientific | 2005 |
| Jenab 2 Limited | Hospitality | 2021 |
| Jenab 3 Limited | Hospitality | 2021 |
| Jenab Limited | Hospitality | 2019 |
| Jenkin & Purser (Holdings) Public Limited Company | | 1957 |
| Jenkin & Purser Limited | | 1949 |
| Jenner Specialist Care and Training Ltd | Health & social care | 2022 |
| Jennerally Speaking Ltd | Professional & scientific | 2023 |
| Jesus SON of Mary Community Centre | Health & social care | 2024 |
| JG Research & Insight Ltd | Professional & scientific | 2020 |
| JH Entertainment Services Ltd | Administrative & support | 2022 |
| Jhag Cleaning Company Ltd | Administrative & support | 2026 |
| Jhony & Marcio Landscapers Ltd | Administrative & support | 2022 |
| Jireh Construction Ltd | Construction | 2019 |
| JJ Commercial Property Limited | Administrative & support | 2016 |
| JJ Group Holdings Limited | Administrative & support | 2022 |
| JJ Lighting Limited | Administrative & support | 2006 |
| JJ Media Agency Ltd | Professional & scientific | 2023 |
| JJ Media Group Ltd | Professional & scientific | 2023 |
| JJ Production Group Limited | Professional & scientific | 2006 |
| JJ Property Investments Limited | Administrative & support | 2016 |
| JJ Studios London Ltd | Professional & scientific | 2023 |
| JJG Events Limited | Arts & recreation | 2017 |
| JL Financial Services Limited | Professional & scientific | 2016 |
| JMP Business Ltd | Real estate | 2024 |
| JOBS4ALL247 Limited | Transport & logistics | 2016 |
| John Laurence Consultancy Limited | Professional & scientific | 2009 |
| John Small & Company Limited | Manufacturing | 1993 |
| Jojo's Boutiques Limited | Wholesale & retail | 2023 |
| Jonathan Lipitch Consulting Ltd | Professional & scientific | 2021 |
| Joon Ltd | Manufacturing | 2023 |
| JPD Rheumatology Services Limited | Health & social care | 2021 |
| JR Pro Build Limited | Construction | 2019 |
| Junegold Ltd | Real estate | 2006 |
| JUNIOR1ON1FOOTBALL Ltd | Other services | 2024 |
| Jupicon Ltd | Information & communication | 2017 |
| Just Works Limited | Construction | 2010 |
| JVT Fitness Limited | Arts & recreation | 2016 |
| JWW Analytics Ltd | Information & communication | 2019 |
| JYB Records Limited | Information & communication | 2024 |
| K Care Plus Ltd | Health & social care | 2021 |
| K S C Builders Limited | Construction | 2016 |
| K W Neuroradiology Ltd | Health & social care | 2019 |
| K3S Pharma Limited | Professional & scientific | 2025 |
| KA Studio Ltd | Administrative & support | 2025 |
| Kaan 1905 Ltd | Other services | 2023 |
| Kaito Ltd | Information & communication | 2024 |
| Kajal Properties Limited | Real estate | 2015 |
| Kalabash Consultancy Ltd | Professional & scientific | 2025 |
| Kalm. Wellbeing Scents Ltd | Manufacturing | 2024 |
| Kamba Properties Limited | Real estate | 2020 |
| Kamso Ltd | Wholesale & retail | 2018 |
| Kanani Property Investments Ltd | Real estate | 2023 |
| Kaprix Powerhouse Logistics Limited | Transport & logistics | 2024 |
| KATT88 Ltd | Hospitality | 2026 |
| Kavita Varsani Limited | Administrative & support | 2018 |
| Kazana Clothing UK Ltd | Wholesale & retail | 2023 |
| Kazmi Consults Limited | Health & social care | 2025 |
| KB Sports Ltd | Wholesale & retail | 2023 |
| Kell CON Limited | Professional & scientific | 2025 |
| Kemoj Ltd | Professional & scientific | 2026 |
| Kemp Place (Bushey) Residents Association Limited | Real estate | 1978 |
| Kendalmoor Ltd | Wholesale & retail | 2013 |
| Kenmore Business Ltd | Real estate | 2019 |
| Kenneth Rice & Sons Rents Limited | Real estate | 1988 |
| Kent Clinical Services Limited | Health & social care | 2018 |
| Keri Barr Architects Ltd | Professional & scientific | 2015 |