| Invista Maintenance Services Ltd | Construction | 2020 |
| IOT Developments Ltd | Construction | 2018 |
| Irepair Anywhere UK Trading Limited | Wholesale & retail | 2023 |
| Isearchlabs Limited | Information & communication | 2026 |
| Isology Ltd | Professional & scientific | 2024 |
| Ispace Software Technology Ltd | Information & communication | 2025 |
| Israela Ltd | Wholesale & retail | 2025 |
| Istha Limited | Professional & scientific | 2026 |
| Isupply Recruitment Ltd | Administrative & support | 2012 |
| IT Caddy Ltd | Information & communication | 2025 |
| IT Skills Consulting Ltd | Information & communication | 2014 |
| Itpm Ltd | Information & communication | 2009 |
| IV Services (Letchworth) Ltd | Wholesale & retail | 2004 |
| Ixtapa Properties Ltd | Real estate | 2017 |
| Iyashi Moon Ltd | Health & social care | 2022 |
| J & C Immigration Ltd | Professional & scientific | 2024 |
| J a Urology Limited | Health & social care | 2019 |
| J C Parker Build Ltd | Construction | 2022 |
| J Charter and a Whyman (Letchworth) Lettings Ltd | Extraterritorial | 2022 |
| J Charter and a Whyman (Letchworth) Limited | Real estate | 2013 |
| J P Sheehan Limited | Professional & scientific | 2013 |
| J V D Property Development Ltd | Construction | 2011 |
| J-T-M Transport Ltd | Transport & logistics | 2018 |
| J. Hyland Motor Services Letchworth Ltd | Wholesale & retail | 2021 |
| J.A.Wilson and Sons(Weston)Limited | Agriculture & fishing | 1954 |
| Jackmans Chippy Ltd | Hospitality | 2023 |
| Jacksolo Media Productions Ltd | Information & communication | 2011 |
| Jackson Global Group Ltd | Administrative & support | 2024 |
| Jackson Global Ltd | Administrative & support | 2013 |
| Jacob Family Investment Company Limited | Professional & scientific | 2018 |
| Jafo Consultancy Ltd | Information & communication | 2022 |
| Jakey Group Ltd | Financial services | 2017 |
| JAM Estates Limited | Real estate | 2018 |
| Janae Ltd | Wholesale & retail | 2025 |
| Janet Brooks Limited | Professional & scientific | 2004 |
| Jarvis Property Ltd | Real estate | 2019 |
| Jaskeerat Limited | Financial services | 2018 |
| JAY Vincze Fine Art Limited | Arts & recreation | 2025 |
| Jaytech (UK) Ltd | Wholesale & retail | 2011 |
| Jazz UP Your Paws Ltd | Other services | 2022 |
| JB Coaching Ltd | Arts & recreation | 2006 |
| Jenny Nordic Ltd | Wholesale & retail | 2021 |
| Jeremy Price Sound Ltd | Information & communication | 2020 |
| JF OPCO1 Ltd | Real estate | 2021 |
| JG Visuals Ltd | Professional & scientific | 2016 |
| JH22LTD Ltd | Wholesale & retail | 2026 |
| JKJ Consulting Ltd | Professional & scientific | 2021 |
| JM Philipps & Partners Electrical Services Ltd | Construction | 2019 |
| JM Relocations Support Ltd | Other services | 2022 |
| JO Murphy Consulting Ltd | Professional & scientific | 2024 |
| John Bent Construction Limited | Construction | 2011 |
| Johnson Tattoos Limited | Other services | 2025 |
| Jomatic Limited | Wholesale & retail | 2019 |
| Jonathan Glazier Media Limited | Administrative & support | 2011 |
| Jones and Brown Properties Ltd | Professional & scientific | 2022 |
| Jones SE Homes Limited | Construction | 2013 |
| Joyce Company PM Limited | Wholesale & retail | 2020 |
| Joyvié Health Ltd | Professional & scientific | 2023 |
| Jpex Consulting Ltd | Information & communication | 2026 |
| Jsat Consulting Limited | Extraterritorial | 2018 |
| JSM Blue Property Management Ltd | Real estate | 2025 |
| JT1 Limited | Wholesale & retail | 2010 |
| Jubilee Trade Centre Limited | Households | 1984 |
| Jugjeet Properties Ltd | Real estate | 2019 |
| Julian Calverley Limited | Professional & scientific | 2006 |
| Jump Advisory Group Limited | Professional & scientific | 2020 |
| Just Like Kids Limited | Education | 2022 |
| Just Resolutions Limited | Professional & scientific | 2011 |
| K Burchell Limited | Professional & scientific | 2014 |
| Kaackai International Ltd | Financial services | 2023 |
| Kamaete Schools of Traditional Shotokan Karate Limited | Professional & scientific | 2018 |
| Kamber Electrical Limited | Construction | 2009 |
| Kanhaiya Limited | Wholesale & retail | 2025 |
| Kanor Chemicals Limited | Extraterritorial | 1983 |
| Karacayir Limited | Hospitality | 2018 |
| Karphi Services Limited | Information & communication | 2008 |
| Kattz (AH) Ltd | Health & social care | 2013 |
| Kattz Kidz Limited | Education | 2016 |
| Kattz RE Ltd | Real estate | 2020 |
| Kawal and Nancy International Limited | Administrative & support | 2004 |
| Kay's Flawless Clean Ltd | Administrative & support | 2026 |
| Keegan Bowman Limited | Professional & scientific | 2023 |
| Kehft’S Pressure and Shine Ltd | Administrative & support | 2025 |
| Kempack Wharf Developments Limited | Manufacturing | 1999 |
| Kemskatering Ltd | Manufacturing | 2025 |
| KEN Mills (Injection) Limited | Manufacturing | 1979 |
| Kendall's Group Ltd | Construction | 2024 |
| Kerti Properties Limited | Real estate | 2016 |
| Key Developments (Godalming) LLP | | 2013 |
| Keyframe Consulting Limited | Professional & scientific | 2019 |
| KGS Communications Ltd | Professional & scientific | 2020 |
| Khoi Khoi Bar and Vino Limited | Hospitality | 2016 |
| King Street Residents Company Limited | Real estate | 2005 |
| KIP Mcgrath Tuition Letchworth LP | | 2017 |
| Kir-Hr Limited | Professional & scientific | 2025 |
| Kittsoft Ltd | Professional & scientific | 2007 |
| KJ Accounting Ltd | Professional & scientific | 2026 |
| Klimov Labs Ltd | Information & communication | 2021 |
| Kmbalstone Ltd | Wholesale & retail | 2021 |
| Knights Field Management (NO. 2) Limited | Households | 1988 |