| J B Davern & Co Ltd | Professional & scientific | 2005 |
| J Locke Services Ltd | Construction | 2022 |
| J P Roofing Services Limited | Construction | 2015 |
| J Richo Financial Services Ltd | Financial services | 2021 |
| J Watson Accountants Ltd | Professional & scientific | 2022 |
| J&S Installations Ltd | Professional & scientific | 2020 |
| J. K. Linceter Ltd | Real estate | 2004 |
| JA Nails Ltd | Other services | 2019 |
| JA Photography Limited | Professional & scientific | 2025 |
| Jacobs Villas Management Company Limited | Households | 2009 |
| Jacs Fitness Ltd | Professional & scientific | 2025 |
| Jadai Ltd | Information & communication | 2025 |
| Jade Country Care Homes Limited | Hospitality | 1998 |
| Jaggard Europe Limited | Construction | 2015 |
| Jake Alun Ltd | Arts & recreation | 2024 |
| James Alexander Properties Limited | Real estate | 2014 |
| James David Land & Estates Ltd | Real estate | 2025 |
| James Squire Ltd | Professional & scientific | 2018 |
| Jamie Coomber Consulting Ltd | Professional & scientific | 2024 |
| Jamjr Limited | Real estate | 2016 |
| Janili Ltd | Real estate | 2024 |
| Jayex Analytics Limited | Information & communication | 2024 |
| Jayex Limited | Information & communication | 2024 |
| Jbskatesservice Ltd | Manufacturing | 2025 |
| JC Talent Solutions Ltd | Professional & scientific | 2021 |
| JCH Global Consulting Limited | Professional & scientific | 2015 |
| Jcktl Ltd | Professional & scientific | 2018 |
| JD Construct Limited | Construction | 2026 |
| JE Psychology Services Limited | Health & social care | 2017 |
| Jeli Media Ltd | Information & communication | 2012 |
| Jeli Services Ltd | Financial services | 2021 |
| Jelmac Developments Limited | Construction | 2011 |
| Jelmac Properties | Real estate | 2008 |
| Jeph Meadway Limited | Financial services | 2025 |
| Jester PVT Ltd | Wholesale & retail | 2019 |
| Jethwa Services Ltd | Administrative & support | 2021 |
| Jevon Homes Limited | Construction | 2010 |
| Jevon Homes [Burston] Limited | Construction | 2004 |
| JGT & Co Ltd | Professional & scientific | 2013 |
| JGT Media Ltd. | Information & communication | 2014 |
| Jh-C Social Ltd | Professional & scientific | 2021 |
| JJT Accounting Limited | Professional & scientific | 2014 |
| JKM Property Group Limited | Real estate | 2025 |
| Jkt'Z Properties Ltd | Real estate | 2020 |
| Jkt'Z Property Services Ltd | Real estate | 2026 |
| Jkt’Z Limited | Real estate | 2020 |
| JL Grady Ltd | Professional & scientific | 2023 |
| JLG Plumbing and Heating Ltd | Construction | 2020 |
| JMS Data Science Solutions Limited | Information & communication | 2025 |
| JN Good Tech Ltd | Wholesale & retail | 2017 |
| JNC Business Solutions Limited | Professional & scientific | 2009 |
| JNG Advisory Ltd | Professional & scientific | 2025 |
| JO Page Consulting Limited | Health & social care | 2016 |
| Joanna Copestake Associates Ltd | Professional & scientific | 2011 |
| John Ellison Electronics Limited | Manufacturing | 2009 |
| John Phillips Education Ltd | Education | 2024 |
| John Slattery Haulage Limited | Real estate | 2005 |
| Johns Sister Holding Ltd | Administrative & support | 2018 |
| Johns Sister Limited | Administrative & support | 2010 |
| Johnston Safety Consulting Limited | Professional & scientific | 2013 |
| Joinery Close MGT Limited | Households | 2024 |
| Joining Careers Education Ltd | Administrative & support | 2025 |
| Joint Communication Solutions Ltd | Information & communication | 2025 |
| Jonadith UK Ltd | Hospitality | 2012 |
| Jones of Berkhamsted Ltd | Real estate | 2018 |
| Jono Events Ltd | Professional & scientific | 2020 |
| JPS Electrical (Berkhamsted) Limited | Construction | 2020 |
| JR Recruitment Specialists Ltd | Administrative & support | 2020 |
| JS Properties and Homes Limited | Real estate | 2012 |
| JS Stone Construction Ltd | Construction | 2022 |
| Julia Eleanor Moodie Ltd | Extraterritorial | 2024 |
| Juliet Michel Interior Design Ltd | Professional & scientific | 2013 |
| Jungle Networks Ltd | Information & communication | 2009 |
| Junglenet Ltd | Professional & scientific | 2009 |
| Juniper Consultants Ltd | Professional & scientific | 2024 |
| JWA Governance Services LLP | | 2012 |
| K&B Professional Services Limited | Professional & scientific | 2023 |
| K1 Health and Wellness Ltd | Arts & recreation | 2018 |
| K7 Yachts Ltd | Manufacturing | 2023 |
| K8I Ltd | Transport & logistics | 2025 |
| KA Nokka Limited | Hospitality | 2023 |
| Kaby Renovations Limited | Administrative & support | 2022 |
| KAD Asset Holdings Ltd | Real estate | 2024 |
| Kadaknath Rare Bird Co. Ltd | Agriculture & fishing | 2025 |
| Kaizen Solutions EU Limited | Wholesale & retail | 2008 |
| Kalm Construct Limited | Construction | 2023 |
| Kana Consult Limited | Professional & scientific | 2021 |
| Kanvas Recruitment Group Ltd | Professional & scientific | 2023 |
| KAR CAM Innovation Ltd | Professional & scientific | 2023 |
| Karen Walls Associates Limited | Professional & scientific | 2001 |
| Karma Designs Limited | Wholesale & retail | 1999 |
| Kate Rennie Media Limited | Information & communication | 2011 |
| Katherine Newton Ltd | Information & communication | 2022 |
| Kathryn E Magson Limited | Real estate | 2009 |
| Katy Regan Ltd | Other services | 2007 |
| KDS Real Estate Limited | Real estate | 2019 |
| Kebab Bros Ltd | Hospitality | 2017 |
| Kelly Commercial Services Limited | Professional & scientific | 2017 |
| Kelly Williams Curtains, Blinds & Soft Furnishings Limited | Other services | 2017 |
| Kensington Properties (EC) Limited | Construction | 1997 |