| Holy Loch Marina Ltd | Real estate | 2025 |
| Hotshots Basketball Ltd | Arts & recreation | 2002 |
| Houseforsale Limited | Other services | 2000 |
| IAN Beveridge Motorsport Limited | Professional & scientific | 2007 |
| Ican Business Systems Limited | Manufacturing | 2003 |
| Ican Technology Limited | Administrative & support | 2005 |
| Icancopy Limited | Administrative & support | 2005 |
| Ilmor Engineering Limited | Manufacturing | 2005 |
| Image I.T. Limited | Manufacturing | 1996 |
| IN Line Duct Cleaning Limited | Administrative & support | 2003 |
| Inferret Limited | Information & communication | 2007 |
| Ingleby (159) | Administrative & support | 1986 |
| Inmoco Limited | Professional & scientific | 1999 |
| Insider Tasting Limited | Education | 2002 |
| Intelligent Motion Control Limited | Professional & scientific | 1987 |
| Inter County Service Group Ltd | Water & waste | 1998 |
| Interior Newman Kamali Ltd | Manufacturing | 2003 |
| International Applications Limited | Wholesale & retail | 2006 |
| Ironstone Homes Limited | Professional & scientific | 2002 |
| IT Accounting Limited | Professional & scientific | 2005 |
| J & P Enterprises (UK) Ltd | Manufacturing | 2003 |
| J Care (UK) Limited | Real estate | 2006 |
| J E S Brickwork Limited | Construction | 2006 |
| J H Glass Ltd | Construction | 2002 |
| J K Mechanical Services Limited | Wholesale & retail | 2003 |
| J Lloyd Limited | Construction | 2002 |
| J Snatchfold Limited | Administrative & support | 2006 |
| J Swane Limited | Construction | 2006 |
| J.A.P. Northampton Limited | Extraterritorial | 1991 |
| J.Rowlatt & Sons Limited | Wholesale & retail | 1918 |
| JAI Cars Northampton Ltd | Transport & logistics | 2023 |
| Jake Mark Limited | Hospitality | 1997 |
| James Murray and Co. (Blenders) Limited | Extraterritorial | 2002 |
| Javor A&D Ltd | Transport & logistics | 2006 |
| JB Pharmatron Limited | Administrative & support | 2005 |
| Jennac Limited | Information & communication | 2005 |
| JJN Consulting Limited | Information & communication | 2002 |
| Jobralke Limited | Professional & scientific | 2007 |
| John Hawkes Limited | Construction | 2003 |
| John Spademan Motor Engineers Limited | Wholesale & retail | 2005 |
| JR Medical Services Ltd | Health & social care | 2024 |
| Junction (Abington) Management Company Limited | Extraterritorial | 2003 |
| Just Electrics Limited | Energy | 2002 |
| K & K Farmers Limited | Financial services | 2004 |
| K Matcham Services Limited | Construction | 2006 |
| K. & N. Waite Construction Limited | Construction | 1995 |
| Kemper (U.K.) Limited | Wholesale & retail | 1996 |
| Kempston Components Limited | Professional & scientific | 1961 |
| Kempston Controls Limited | Professional & scientific | 2002 |
| Kettering Association for Mental Health | Health & social care | 1998 |
| Kettering Community Business Centre Development Trust | Real estate | 1999 |
| Kettering Flat Roofing Ltd | Construction | 2021 |
| Keystone Plastics Limited | Extraterritorial | 2006 |
| Keystone Roofing & Facades Limited | Construction | 2003 |
| Keystone Roofing Supplies Limited | Wholesale & retail | 2002 |
| Ki-Tran Limited | Transport & logistics | 2008 |
| Kingfisher Marinas Limited | Other services | 1998 |
| Kiziak Group Holdco Ltd | Financial services | 2024 |
| KJL Steels (UK) Limited | Wholesale & retail | 2000 |
| KP Consultancy Services Ltd | Professional & scientific | 2003 |
| L.D. Fabrications Limited | Manufacturing | 2001 |
| LA Holistic Health & Natural Beauty Clinics Ltd | Health & social care | 2023 |
| Lawrie Ltd | Hospitality | 2022 |
| Lazeme Ltd | Wholesale & retail | 2007 |
| Legersense Limited | Professional & scientific | 1985 |
| Lenex Limited | Energy | 2015 |
| Let's Rent Property Ltd | Real estate | 2002 |
| LGS Brickwork Limited | Construction | 2004 |
| LHG1 Limited | Health & social care | 2017 |
| Liberty Medical Services Limited | Wholesale & retail | 2019 |
| Lifechoice Homes Ltd | Other services | 2017 |
| Light Aircraft Association Limited | Other services | 1958 |
| Lilford Lodge Lakes Limited | Energy | 2005 |
| Listbasic Limited | Wholesale & retail | 1991 |
| LMH (Civil Engineering) Limited | Construction | 1996 |
| Lodge Farm Properties Limited | Agriculture & fishing | 2023 |
| London Educational Solutions Ltd | Administrative & support | 2024 |
| Long Buckby Trading Ltd | Wholesale & retail | 2020 |
| Longbow Computing Limited | Information & communication | 2004 |
| LRN 2 Drve Limited | Education | 2007 |
| Lucas Estate Agents Limited | Real estate | 2003 |
| Luke Tear Limited | Construction | 2006 |
| Lumsden Housing | Other services | 2005 |
| Lunar Consulting Limited | Professional & scientific | 1998 |
| M J S Systems Development Limited | Information & communication | 1996 |
| M K Property Care (Management) Limited | Real estate | 1996 |
| M Kilsby Limited | Construction | 2004 |
| M on the Square Limited | Hospitality | 2020 |
| M.A. Technologies Limited | Information & communication | 1997 |
| M.E.R.G. Limited | Real estate | 1993 |
| M.L. Wakefield Limited | Wholesale & retail | 2001 |
| M.P.B. Structures Ltd | Construction | 1986 |
| Major Diamond Supplies Limited | Administrative & support | 2007 |
| Mander's Paints Limited | Wholesale & retail | 2001 |
| Mandies Pantry Limited | Hospitality | 2002 |
| Mantis Cranes Limited | Wholesale & retail | 2006 |
| Manus Estates Limited | Construction | 1987 |
| Margolis Civils Ltd | Construction | 2019 |
| Mark Moore Limited | Construction | 2003 |
| Mark Ryan Consulting Limited | Information & communication | 2001 |