| Internet Results Limited | Information & communication | 2006 |
| Into Deep Limited | Information & communication | 2023 |
| Intraweb Services Limited | Information & communication | 2019 |
| Intraweb Systems Ltd | Information & communication | 2004 |
| Inutec Limited | Water & waste | 2002 |
| Inveon Digital Commerce Solutions Ltd | Information & communication | 2018 |
| IPP Management & Design Ltd | Real estate | 2008 |
| Ipulse Systems Limited | Information & communication | 2014 |
| Irepair Solution Ltd | Wholesale & retail | 2015 |
| Iris IS Digital Limited | Professional & scientific | 2025 |
| Ispira Solutions Limited | Professional & scientific | 2008 |
| IVY Select Ltd | Information & communication | 2025 |
| J a Business Consulting Services Ltd | Information & communication | 2010 |
| J M Preston Limited | Wholesale & retail | 2018 |
| J&Bl Services Ltd | Other services | 2013 |
| J. Janeiro (UK) Limited | Real estate | 1985 |
| J.E.S. Consultancy Ltd | Professional & scientific | 2018 |
| J2 Design Consultants Limited | Professional & scientific | 2007 |
| JAC Distributors Ltd | Wholesale & retail | 2018 |
| Jack Design & Build Ltd | Construction | 2026 |
| Jacp Investments Ltd | Financial services | 2024 |
| Jacu Ltd | Wholesale & retail | 2011 |
| Jacux Ltd | Wholesale & retail | 2025 |
| Jala Ltd | Other services | 2018 |
| Jalawi Limited | Arts & recreation | 2025 |
| James Hick Services Ltd | Professional & scientific | 2025 |
| James Marc Contracting Limited | Construction | 2025 |
| James Marc Properties Ltd | Real estate | 2021 |
| Jameson Event Storage Limited | Transport & logistics | 2017 |
| Jane Chesters Limited | Professional & scientific | 2012 |
| Jargonjar Limited | Information & communication | 2026 |
| Jaycee Tools & Machinery Limited | Wholesale & retail | 1973 |
| JBF1987 Ltd | Arts & recreation | 2017 |
| JDB Limited | Professional & scientific | 2024 |
| JDC Newco Ltd | Financial services | 2025 |
| JDF Electrical Ltd | Construction | 2018 |
| JDR Development Solutions Limited | Information & communication | 2011 |
| Jedi Developments Limited | Construction | 2019 |
| JEDIPILOT1 Limited | Professional & scientific | 2019 |
| Jejd Property Limited | Hospitality | 2021 |
| Jelly Labs AI Ltd | Information & communication | 2026 |
| Jericos Heating Ltd | Construction | 2022 |
| Jerid Limited | Professional & scientific | 2016 |
| Jfreaky-Lvlup Limited | Arts & recreation | 2024 |
| JHC Ventures Ltd | Financial services | 2016 |
| Jigsaw Accounting Ltd | Professional & scientific | 2024 |
| JKM Marlow Limited | Financial services | 2021 |
| JLB Vintage Ltd | Wholesale & retail | 2026 |
| JMB Horsepower (UK) Ltd | Wholesale & retail | 2009 |
| JNA Property Limited | Real estate | 2006 |
| Jobrouter Ltd | Information & communication | 2016 |
| Johanson Dielectrics Limited | Administrative & support | 1998 |
| John Darvell Packaging Limited | Wholesale & retail | 1976 |
| Johnson Walker Limited | Wholesale & retail | 1989 |
| Jonathan Planner Associates Limited | Administrative & support | 2010 |
| Jonathan Smith Consultants Ltd | Professional & scientific | 2019 |
| JP Stylist Ltd | Professional & scientific | 2025 |
| JPM Management Ltd | Professional & scientific | 2018 |
| JPP Construction Limited | Construction | 2025 |
| JR Davidson Marketing Limited | Professional & scientific | 2025 |
| JR Luxury Limited | Manufacturing | 2012 |
| JRB Contractors (London) Ltd | Construction | 2023 |
| Juice Locations UK Ltd | Hospitality | 2021 |
| Juniper Court (Flackwell Heath) Management Company Limited | Households | 2003 |
| Jupiter Hotels Holdings Limited | Administrative & support | 2011 |
| Just Smith Ltd | Administrative & support | 2016 |
| Justklouds Ltd | Information & communication | 2023 |
| Jvna Investments Limited | Financial services | 2021 |
| K&J Hunter Ltd | Real estate | 2026 |
| K'S Bicycle Workshop Limited | Manufacturing | 2017 |
| K2 Property Group Limited | Real estate | 2022 |
| Kaleidoscopes Group Limited | Professional & scientific | 2024 |
| Kalincevic Limited | Other services | 2008 |
| Karavan Journey Limited | Hospitality | 2023 |
| Kasei Ltd | Extraterritorial | 2023 |
| Kate Mcneil Photography Limited | Professional & scientific | 2015 |
| Kazei Group Limited | Professional & scientific | 2023 |
| Kazei Limited | Extraterritorial | 2023 |
| KCD Ventures Limited | Financial services | 2024 |
| KD (Ckaf) Ltd | Hospitality | 2022 |
| KD Film Consultants Ltd | Professional & scientific | 2024 |
| Keenmar Investments Ltd | Real estate | 2023 |
| Keenway Limited | Construction | 2013 |
| Kelly Gomm Consulting Limited | Administrative & support | 2014 |
| Kelly IT Contracting Ltd | Information & communication | 2026 |
| Kelvin House Management Company Limited | Households | 2019 |
| Kensington Mortgage Company Limited | Financial services | 1995 |
| Kensington Mortgage Services Limited | Administrative & support | 2020 |
| Kerriland Limited | Households | 1987 |
| Kerry Fretwell Limited | Professional & scientific | 2024 |
| Kerry's Gifts Limited | Other services | 2018 |
| Keswick Court Management Company (Slough) Limited | Households | 1983 |
| Key Network Services Ltd | Information & communication | 2006 |
| KG Real Estate Limited | Real estate | 2020 |
| KHM Hospitality Ltd | Hospitality | 2025 |
| Kikkar Health Club - Loudwater Ltd | Arts & recreation | 2026 |
| Kikkar Properties Limited | Real estate | 2026 |
| Kiko Car Service Limited | Transport & logistics | 2018 |
| Kiltegan Coffee Ltd | Manufacturing | 2021 |
| Kimiki Ventures Limited | Other services | 2025 |