| IVY Hospitality UK Limited | Hospitality | 2025 |
| Ivybeau Homes Ltd | Real estate | 2025 |
| Ivygate Palmerston Road Management Company Limited | Real estate | 2004 |
| IX5 Environmental Services Ltd | Agriculture & fishing | 2022 |
| IXC Technologies Ltd | Information & communication | 2026 |
| Izak Ltd | Administrative & support | 2018 |
| Izzy KAY Properties Limited | Real estate | 2024 |
| J & a Cars (Northampton) Limited | Wholesale & retail | 2014 |
| J & C Bluebells Limited | Real estate | 2025 |
| J & D Goodridge Building Services Limited | Construction | 2018 |
| J & E Clegg Limited | Agriculture & fishing | 2019 |
| J & G Rees & Co Ltd | Construction | 2019 |
| J & J Lewis Property Limited | Real estate | 2024 |
| J & J Paving Limited | Construction | 2020 |
| J & K Scaffolding (Northampton) Limited | Construction | 1999 |
| J & M Harding Developments Ltd | Construction | 2023 |
| J & N Plumbing and Heating Limited | Construction | 2019 |
| J & N Weaver Properties Limited | Real estate | 2022 |
| J & R’S Agricultural Limited | Agriculture & fishing | 2024 |
| J & S Potter Ltd | Manufacturing | 2013 |
| J & S Wright Ltd | Real estate | 2017 |
| J &DOR Management Ltd | Manufacturing | 2023 |
| J a Burton Tiling Contractors Limited | Construction | 2013 |
| J a Taylor Ltd | Construction | 2021 |
| J a Taylor Properties Ltd | Real estate | 2015 |
| J B Carpentry & Kitchens Ltd | Construction | 2015 |
| J B House Limited | Construction | 2009 |
| J B J Machines Limited | Real estate | 1980 |
| J B S Hardware Limited | Administrative & support | 1988 |
| J Brown Consulting Ltd | Financial services | 2012 |
| J Burgess Heating Services Ltd | Construction | 2026 |
| J C H Logistics Limited | Transport & logistics | 2012 |
| J C Motor Engineers Ltd | Other services | 2010 |
| J Curtis Development Limited | Construction | 2024 |
| J D Garage Limited | Wholesale & retail | 2009 |
| J Doonan Formwork Ltd | Construction | 2023 |
| J E Property Investments Limited | Real estate | 2019 |
| J F a Management Ltd | Professional & scientific | 2023 |
| J Greaves Ltd | Information & communication | 2023 |
| J Ingle Maintenance Ltd | Construction | 2026 |
| J J Plumbing Services (2008) Ltd | Construction | 2008 |
| J L and Company Limited | Manufacturing | 1982 |
| J L Hill Carpets Limited | Wholesale & retail | 2018 |
| J M Bygate Designs Limited | Professional & scientific | 2008 |
| J M Fairhurst Design Ltd | Professional & scientific | 2017 |
| J M Scaffolding Ltd | Construction | 1999 |
| J Mayhew Automotive Ltd | Wholesale & retail | 2025 |
| J Miller Consulting Ltd | Information & communication | 2011 |
| J Monarch Ltd | Wholesale & retail | 2026 |
| J Newitt & Sons Ltd | Wholesale & retail | 2009 |
| J P Builders UK Ltd | Construction | 2017 |
| J P Contractors Limited | Construction | 1996 |
| J P Property Management Limited | Real estate | 2007 |
| J P Weaver Consulting Limited | Professional & scientific | 2019 |
| J R Bentley Limited | Professional & scientific | 2020 |
| J R Market Research Limited | Professional & scientific | 2025 |
| J S Peer Limited | Construction | 2003 |
| J Smith Electrical Ltd | Extraterritorial | 2012 |
| J Sweeney Accountants Limited | Professional & scientific | 2016 |
| J T Butts Limited | Information & communication | 2026 |
| J T Hemming Dental Laboratories Limited | Manufacturing | 2023 |
| J T Holman Building Contractors Limited | Construction | 2024 |
| J V Logistics Limited | Transport & logistics | 2012 |
| J Vads Ltd | Wholesale & retail | 2024 |
| J W Kennedy & Partners Ltd | Professional & scientific | 2009 |
| J White Consulting Ltd | Professional & scientific | 2025 |
| J&H Motors Ltd | Wholesale & retail | 2022 |
| J&J Bros Ltd | Real estate | 2025 |
| J&J PRE Ltd | Construction | 2026 |
| J&Ko Property Limited | Real estate | 2014 |
| J&N Fast Drop Limited | Wholesale & retail | 2022 |
| J&S Autos (Northampton) Limited | Wholesale & retail | 2011 |
| J&S Capital Property Investment Limited | Real estate | 2025 |
| J&S Property Innovations Ltd | Real estate | 2025 |
| J&T King Consulting Limited | Professional & scientific | 2017 |
| J'Aime Shoes Ltd | Wholesale & retail | 2025 |
| J'Nik's Limited | Hospitality | 2018 |
| J'S Cash and Carry Limited | Wholesale & retail | 2022 |
| J-Cube Properties Ltd | Real estate | 2023 |
| J-Tech Solutions LLP | | 2003 |
| J-Walk IP Ltd | Professional & scientific | 2021 |
| J-Walk Ltd | Professional & scientific | 2003 |
| J-Walk Property Ltd | Real estate | 2021 |
| J. a. Knight & SON (Farmers) Limited | Agriculture & fishing | 1968 |
| J. Cooke Roofing Ltd | Construction | 2002 |
| J. Scott (Thrapston) Limited | Construction | 1946 |
| J. T. Bevann DRY Lining Specialists Limited | Construction | 1985 |
| J.A Bespoke Consulting Limited | Professional & scientific | 2023 |
| J.A.K PUB Ltd | Hospitality | 2025 |
| J.A.K. Roofing Limited | Construction | 2014 |
| J.A.K. Roofing Supplies Limited | Wholesale & retail | 2014 |
| J.B. Marketing Limited | Wholesale & retail | 1991 |
| J.B.E Consultancy Limited | Financial services | 2006 |
| J.C & S.E Tate Investments Limited | Construction | 2017 |
| J.C.L. (Investments) Limited | Real estate | 1975 |
| J.C.S. Computing Limited | Information & communication | 1988 |
| J.D.B. Industrial Roofing Limited | Other services | 2017 |
| J.E.J Contracting Limited | Agriculture & fishing | 2023 |
| J.E.Matthews and Sons Limited | Manufacturing | 1952 |
| J.E.T. Dove Limited | Real estate | 2022 |