| J B Mothersole & Associates LLP | | 2015 |
| J Clarke Building Services Limited | Construction | 2013 |
| J D Wyatt Contracting Limited | Information & communication | 2019 |
| J E Hine Limited | Information & communication | 2023 |
| J G Home Improvements Ltd | Construction | 2020 |
| J Higgins Consultancy Ltd | Public administration | 2025 |
| J Kincaid Limited | Wholesale & retail | 2007 |
| J L White Engineering Limited | Manufacturing | 2025 |
| J N Solutions Limited | Professional & scientific | 1999 |
| J R Laxon & C J Miller Limited | Administrative & support | 2003 |
| J R Whittome Ltd | Financial services | 2010 |
| J Rose Ltd | Information & communication | 2018 |
| J S Aviation Adjusting Limited | Financial services | 2015 |
| J Whitford Limited | Construction | 2018 |
| J.V. Halsey Civil Engineering Limited | Construction | 2008 |
| Jacob J James Limited | Real estate | 2020 |
| Jacob's Ladder Consulting Limited | Professional & scientific | 2013 |
| Jacobiyan Consulting Ltd | Information & communication | 2025 |
| Jacqueline Burns Ltd | Extraterritorial | 2018 |
| JAD Consults Ltd | Information & communication | 2020 |
| Jaffer Consulting Limited | Information & communication | 2021 |
| Jaime Brown Ltd | Real estate | 2014 |
| Jake Gower Limited | Professional & scientific | 2025 |
| James & Austin Accountancy Services Ltd | Professional & scientific | 2025 |
| James Holmes Planning Ltd | Professional & scientific | 2022 |
| Janewayfx Ltd | Information & communication | 2026 |
| Jansel Square Management Limited | Real estate | 1990 |
| Jaques PR Limited | Professional & scientific | 2018 |
| Jarm Consultancy Ltd | Administrative & support | 2024 |
| Jays Gardens Ltd | Administrative & support | 2024 |
| JB Doran Consulting Ltd | Information & communication | 2022 |
| JB Fire Limited | Professional & scientific | 2016 |
| Jbay Consultancy Ltd | Information & communication | 2023 |
| JC Primus Ltd | Arts & recreation | 2018 |
| JCT Property Limited | Real estate | 2020 |
| JD Practical Ltd | Construction | 2011 |
| JDA Build Limited | Construction | 2016 |
| Jdpb Limited | Information & communication | 2017 |
| JE Portfolio Ltd | Extraterritorial | 2024 |
| JE Pubs Ltd | Hospitality | 2023 |
| JE Tailoring Ltd | Wholesale & retail | 2020 |
| Jeap Data Services Ltd | Information & communication | 2024 |
| Jeff Bond Ltd | Health & social care | 2011 |
| Jemma Scott Property Limited | Real estate | 2026 |
| Jerome Property Ltd | Real estate | 2021 |
| Jess Brooks Limited | Administrative & support | 2014 |
| JGT Broadcast Limited | Information & communication | 2026 |
| Jhough Consulting Ltd | Information & communication | 2015 |
| JIG Tailor Ltd | Professional & scientific | 2025 |
| Jilly Norris PT Ltd | Other services | 2024 |
| JJ Sports Bar Ltd | Real estate | 2009 |
| JK Contracting (Flight) Ltd | Information & communication | 2021 |
| JL Flooring Ltd | Construction | 2019 |
| JLB Transactions Limited | Real estate | 2022 |
| JMD Consultancy Ltd | Information & communication | 2024 |
| Jmen Limited | Professional & scientific | 2018 |
| JND Installations Ltd | Construction | 2021 |
| JOE Brimson Ltd | Professional & scientific | 2024 |
| Joe's Vintage Coffee Van Limited | Hospitality | 2016 |
| Joel J Wilson Ltd | Information & communication | 2025 |
| John Keay Limited | Construction | 2018 |
| Johns Cars of Tring Limited | Transport & logistics | 2024 |
| Joint Ventures (GB) Ltd | Administrative & support | 2010 |
| Jolies Choses Ltd | Professional & scientific | 2023 |
| Josefsberg Ltd | Real estate | 2013 |
| Journal of Biophilic Design Limited | Information & communication | 2024 |
| JPM Building&Landscaping Ltd | Administrative & support | 2022 |
| JSB Consulting Limited | Professional & scientific | 2019 |
| JSP Creative Limited | Information & communication | 2020 |
| JT Cleaning Company Ltd | Administrative & support | 2025 |
| JT Digital Consulting Ltd | Information & communication | 2024 |
| JTL Scenery Ltd | Manufacturing | 2022 |
| Jubilee Gardens (Tring) Management Limited | Administrative & support | 2012 |
| Julia Crane Psychology Ltd | Education | 2018 |
| Jungle Beat Limited | Information & communication | 2008 |
| JUST2CONNECT Limited | Information & communication | 2009 |
| JW Insightful Analysis Ltd | Information & communication | 2018 |
| K C Mouldings Ltd | Construction | 2013 |
| K Lynch Process Consultancy Ltd | Professional & scientific | 2018 |
| K&J Real Estate Ltd | Real estate | 2026 |
| K&M Productions Ltd | Information & communication | 2024 |
| Kaans Computing Ltd | Other services | 2020 |
| Kagb Ltd | Real estate | 2024 |
| Kandola Consultancy Ltd | Information & communication | 2014 |
| Kanoppi Limited | Information & communication | 2022 |
| Kavanagh Mortgage Financial Ltd | Financial services | 2020 |
| Kavoney Consulting Limited | Information & communication | 2016 |
| KAY Pictures Limited | Health & social care | 2010 |
| Kaybee Technology Limited | Information & communication | 2024 |
| Kayleigh Watts Equestrian Ltd | Education | 2026 |
| KC WEX Training Ltd | Education | 2021 |
| Kdgh Solutions Limited | Information & communication | 2023 |
| Keep Accounting Ltd | Professional & scientific | 2017 |
| Keirodesign Limited | Information & communication | 2018 |
| Kelake Ltd | Professional & scientific | 2018 |
| Kennealy Bespoke Ltd | Manufacturing | 2024 |
| Kevin Willis Legal and Data Privacy Limited | Professional & scientific | 2019 |
| KEW Consultancy Ltd | Administrative & support | 2011 |
| KEW Creative Limited | Administrative & support | 2003 |
| Key Land Estates Architecture Ltd | Professional & scientific | 2020 |