| Islianda Construction Consultancy Limited | Professional & scientific | 2025 |
| Islip Power Distribution Limited | Professional & scientific | 2021 |
| Iswid Limited | Manufacturing | 2025 |
| IT Assure Limited | Information & communication | 2015 |
| IT Starts With an Idea Ltd | Professional & scientific | 2024 |
| ITM Electrical Ltd | Construction | 2023 |
| Iuvo Events & Project Management Ltd | Administrative & support | 2010 |
| Ivan Catering Services Ltd | Hospitality | 2022 |
| Ivara Limited | Information & communication | 2025 |
| IVY Mortgages Ltd | Financial services | 2025 |
| IWA Transport Ltd | Transport & logistics | 2018 |
| Izabela Dickens Coaching Ltd | Health & social care | 2022 |
| Izador Limited | Wholesale & retail | 2019 |
| Izel Luxe Ltd | Wholesale & retail | 2025 |
| J & a Russo Ltd | Real estate | 2022 |
| J & D Mobility Services Limited | Wholesale & retail | 1991 |
| J & J Civil Engineering Ltd | Construction | 2019 |
| J & J Electrics Limited | Construction | 2016 |
| J & K Packaging Ltd | Manufacturing | 2023 |
| J & T Lexington Services Limited | Professional & scientific | 2012 |
| J B Commercial Interiors Ltd. | Wholesale & retail | 2005 |
| J Blanchard Limited | Construction | 2015 |
| J C Audio Visual Limited | Construction | 2026 |
| J D Lawrence Electrical Limited | Construction | 2017 |
| J D Warren Property Limited | Real estate | 2026 |
| J G Mandeville Ltd | Professional & scientific | 2010 |
| J G Wood Limited | Construction | 2014 |
| J Harwood Project Management Limited | Professional & scientific | 2015 |
| J J a Property Developments Ltd | Construction | 2025 |
| J L Clad Ltd | Construction | 2020 |
| J Lewis Enterprises Ltd | Wholesale & retail | 2019 |
| J Lyon Consulting Ltd | Professional & scientific | 2025 |
| J Penney Limited | Construction | 2019 |
| J Quinn (Groundworks) Limited | Construction | 2014 |
| J R Heath Plumbing & Heating Ltd | Construction | 2015 |
| J S C and SON Services Ltd | Construction | 2021 |
| J S Collections Ltd | Administrative & support | 2009 |
| J T Markham Engineering Ltd | Wholesale & retail | 2022 |
| J W Scaffolding (Northants) Limited | Administrative & support | 2015 |
| J Wood Tree Services Ltd | Agriculture & fishing | 2023 |
| J&B Plastering Services Limited | Construction | 2018 |
| J&J Health Care Solutions Ltd | Health & social care | 2024 |
| J&T UK Brickwork Ltd | Construction | 2021 |
| J. Memoli (Electrical Installations) Limited | Manufacturing | 1999 |
| J.R. Domestic Window Cleaning Limited | Administrative & support | 2008 |
| J.S.T. Forklifts Limited | Wholesale & retail | 1973 |
| J.S.Transport Inc Limited | Transport & logistics | 2012 |
| J316 Home Solutions Ltd | Real estate | 2026 |
| J@Maxwell Limited | Information & communication | 2014 |
| Ja&D Care Company Ltd | Health & social care | 2026 |
| Jaap Estates Limited | Real estate | 2021 |
| Jaap Investments Limited | Real estate | 2022 |
| Jaap Realtors Limited | Real estate | 2024 |
| Jabrro Trans Ltd | Transport & logistics | 2018 |
| Jacaranda Properties Ltd | Real estate | 2025 |
| Jacksta Energy Limited | Wholesale & retail | 2010 |
| Jacob Thompson Electrical Ltd | Professional & scientific | 2025 |
| Jade Windows and Doors (Installations) Ltd | Construction | 2022 |
| Jade Windows and Doors Ltd | Construction | 2014 |
| Jagard Valeting & Cleaning Services Ltd | Other services | 2019 |
| Jagga Construction Ltd | Construction | 2026 |
| Jaji Munjeri Limited | Health & social care | 2016 |
| JAK Enterprises Limited | Administrative & support | 1991 |
| Jamburi Events CIC | Arts & recreation | 2026 |
| James a G Smith Consulting Ltd | Manufacturing | 2023 |
| James Reed Brickwork Limited | Construction | 2022 |
| JAN Dohnalek Planning Ltd | Construction | 2018 |
| Jane COE Properties Limited | Extraterritorial | 2016 |
| Jane Smith Beauty Limited | Other services | 2020 |
| Janet Marcia Whitehead Limited | Extraterritorial | 2025 |
| Janki Shiv Ltd | Real estate | 2025 |
| Janki Thania Limited | Real estate | 2020 |
| Jannah Indian Cuisine Limited | Hospitality | 2016 |
| Janvi and Jaishvi Ltd | Real estate | 2023 |
| Japal Property Services Limited | Real estate | 2025 |
| Jarnold & Co Ltd | Real estate | 2020 |
| Jarob Limited | Transport & logistics | 2014 |
| Jashma Ltd | Wholesale & retail | 2024 |
| Jasmine Court (Northampton) Management Limited | Real estate | 2005 |
| Jason Read Specialist Plastering Limited | Construction | 2019 |
| Jatin Patel Limited | Information & communication | 2025 |
| Jaybeam Limited | Manufacturing | 1971 |
| Jaykel Interiors Limited | Other services | 2006 |
| Jaykom Ltd | Wholesale & retail | 2017 |
| Jaykum LLP | | 2011 |
| Jayla Group Ltd | Hospitality | 2024 |
| Jays Autocentre Ltd | Wholesale & retail | 2015 |
| JB Developments (Wentworth) Limited | Construction | 2020 |
| JBS Consultancy (Northants) Limited | Professional & scientific | 2024 |
| JC Bookkeeping Solutions Ltd | Professional & scientific | 2015 |
| JC Construction South West Ltd | Construction | 2026 |
| JC Haulage Service Ltd | Transport & logistics | 2019 |
| JC JEE Ltd | Real estate | 2026 |
| JC Railways Ltd | Construction | 2018 |
| Jchs Engineering Ltd | Construction | 2019 |
| JD Cash and Carry Limited | Wholesale & retail | 2025 |
| JD Motorsport Consultancy Ltd | Professional & scientific | 2025 |
| Jd&C Consultancy Ltd | Information & communication | 2020 |
| JDE Refreshments Ltd | Hospitality | 2026 |
| JDI Services Limited | Other services | 2023 |