| Jameson Brown Limited | Manufacturing | 2000 |
| Janus Financial Consultancy Ltd | Financial services | 2004 |
| Jarmon Limited | Real estate | 2000 |
| Jasonpix Limited | Other services | 2014 |
| Javlin Estates Ltd | Real estate | 2019 |
| Javlin Homes (the Stables) Ltd | Construction | 2025 |
| Javlin Homes Limited | Construction | 2002 |
| JB Auto Guard Limited | Administrative & support | 2024 |
| JC Clinical Services Limited | Wholesale & retail | 2024 |
| JC Logistic Solutions Limited | Transport & logistics | 2016 |
| JD Hawes & SON Limited | Administrative & support | 2012 |
| Jefferyes Property Services Ltd | Real estate | 2018 |
| JEM Complete Building Services Limited | Construction | 2013 |
| Jengu Ltd | Health & social care | 2022 |
| Jennifer Cornish Design Limited | Professional & scientific | 2023 |
| Jeremy Harris Limited | Transport & logistics | 2002 |
| JHH Distribution Ltd | Wholesale & retail | 2025 |
| JHH Transport & Towing Ltd | Transport & logistics | 2025 |
| Jj's Caribbean Ltd | Hospitality | 2026 |
| JKF Controls Limited | Construction | 2017 |
| JM Bathrooms and Design Ltd | Construction | 2021 |
| JMG Store Ltd | Wholesale & retail | 2018 |
| JMH Fencing Ltd | Construction | 2026 |
| JMH Housing Ltd | Real estate | 2022 |
| Jmkl Ltd | Information & communication | 2024 |
| JMS Stockwell Ltd | Construction | 2016 |
| Jojokim Cleaning Services Ltd | Administrative & support | 2025 |
| Jonathon Turner Developments Ltd | Construction | 2022 |
| Journey to Calm Ltd | Information & communication | 2019 |
| JP Beni Consulting Ltd | Professional & scientific | 2020 |
| JPL Films Ltd | Arts & recreation | 2020 |
| JSS Fencing Limited | Administrative & support | 2019 |
| JT Nexus Ltd | Administrative & support | 2013 |
| JTB Direct Ltd | Wholesale & retail | 2021 |
| Jubilee Commercial Services Limited | Extraterritorial | 1996 |
| Jubilee Hire (2001) Limited | Professional & scientific | 2001 |
| Juniper School Ltd | Education | 2022 |
| Jupe Property Services Ltd | Construction | 2017 |
| Justaps Solutions Limited | Manufacturing | 2012 |
| K B Patel & Co Ltd | Wholesale & retail | 2020 |
| K C Real Estate Limited | Real estate | 2012 |
| K R H Electrical Ltd | Construction | 2018 |
| K Smith (PH) Limited | Hospitality | 2022 |
| K&K Nails Bar Ltd | Other services | 2025 |
| Kabali DA Ltd | Real estate | 2021 |
| Kane Sanderson Miles Limited | Real estate | 2016 |
| KARS-4-U Limited | Wholesale & retail | 2016 |
| Karsrus Ltd | Administrative & support | 2025 |
| Katielee Catering Limited | Hospitality | 2003 |
| KC Infotech Ltd | Information & communication | 2016 |
| Keeef UK Limited | Professional & scientific | 2019 |
| Kemmuna Limited | Wholesale & retail | 2018 |
| Kempley Group Holdings Ltd | Professional & scientific | 2025 |
| Kennet Heath Management Limited | Construction | 2015 |
| Kennet Horse Boat Company Limited | Arts & recreation | 2003 |
| Kennet Landscape Solutions Ltd | Administrative & support | 2015 |
| Kennet Service Ltd | Transport & logistics | 2022 |
| Kevin Billings Services Limited | Professional & scientific | 2017 |
| Kevin Rooney Ltd | Professional & scientific | 2011 |
| KG & J Aldridge Limited | Professional & scientific | 1995 |
| Khan Medical and Information Services Ltd | Health & social care | 2020 |
| Khorovats Ltd | Information & communication | 2024 |
| Kierish (Bracknell) Limited | Construction | 2018 |
| Kilbride Murphy Management Limited | Real estate | 2016 |
| Kilroy Electrical Ltd | Construction | 2021 |
| Kimplas Limited | Wholesale & retail | 2001 |
| Kinghorn Properties Limited | Real estate | 1954 |
| Kingsmeadow Management Company Limited | Households | 2000 |
| Kirana Collective Ltd | Wholesale & retail | 2025 |
| Kiss the Red Limited | Information & communication | 2011 |
| Kite Accountancy Ltd | Professional & scientific | 2018 |
| Klavsm Limited | Construction | 2019 |
| Klone Consulting Limited | Professional & scientific | 2017 |
| Kmcg Consulting Limited | Professional & scientific | 2010 |
| Kmmlb Limited | Information & communication | 2014 |
| Knight Global Security Services Ltd | Administrative & support | 2025 |
| Knightsbridge Mechanical Handling Limited | Administrative & support | 2004 |
| Knit 24 Limited | Extraterritorial | 2005 |
| Knox Applied Security Ltd | Information & communication | 2017 |
| Koeneurope Ltd | Construction | 2014 |
| Koenigsberg REI Ltd | Real estate | 2023 |
| KRJ Foods Ltd | Manufacturing | 2025 |
| Kurb Group Ltd | Construction | 2026 |
| Kurvis Limited | Wholesale & retail | 2009 |
| KW Transformation Solutions Ltd | Information & communication | 2025 |
| KWL Commercial Cleaning Limited | Administrative & support | 2025 |
| L J Graham Ltd | Construction | 2023 |
| LA Pizza Bella Ltd | Hospitality | 2025 |
| Ladygrove Ltd | Health & social care | 2026 |
| Lafone Communications Limited | Information & communication | 2012 |
| LAH Asset Management Ltd | Extraterritorial | 2016 |
| Laja Homes Ltd | Real estate | 2021 |
| Lamb Arts Gallery Ltd | Arts & recreation | 2014 |
| Land Properties London Limited | Real estate | 2015 |
| Landmark Roofing & Guttering Ltd | Construction | 2026 |
| Lani & Larsa Limited | Administrative & support | 2024 |
| Laurabella Cosmetics Ltd | Wholesale & retail | 2023 |
| Laurabella Limited | Health & social care | 2017 |
| Lauren of Clerkenwell Limited | Arts & recreation | 2023 |
| Leader Electronics of Europe Limited | Manufacturing | 2005 |