| Infinity Financial Consulting Ltd | Professional & scientific | 2015 |
| Information Flow Limited | Information & communication | 1994 |
| Information Security Systems Limited | Construction | 2003 |
| Information Technology Support Associates (UK) Ltd | Other services | 2002 |
| Inline K9 UK II Dog Training Limited | Agriculture & fishing | 2021 |
| Inline Moves of Maidenhead Limited | Transport & logistics | 1996 |
| Inspectorate of the Security Industry Limited | Administrative & support | 1982 |
| Integrative Cognitive Solutions Limited | Education | 2002 |
| Interact Associates Ltd | Professional & scientific | 2000 |
| Interact Europe 1993 Ukeig | | 1993 |
| Interceptor Communications Limited | Professional & scientific | 1989 |
| Interim.Team Ltd | Information & communication | 2013 |
| Interlease Limited | Households | 1981 |
| Intersurgical UK Holdco Ltd | Financial services | 2026 |
| Ionic Consultancy Limited | Professional & scientific | 1992 |
| IPT Group (UK & I) Limited | Information & communication | 2004 |
| Island Amazon Adventure Limited | Arts & recreation | 1992 |
| IT Value Services Ltd | Information & communication | 2005 |
| Ithaca Solutions Limited | Professional & scientific | 1999 |
| ITS (Bristol) Ltd | Administrative & support | 2000 |
| ITS (Technical Recruitment) Ltd | Administrative & support | 2007 |
| J & K P Enterprises Limited | Professional & scientific | 2005 |
| J C Rigging Ltd | Construction | 2010 |
| J H Building Finishers Ltd | Administrative & support | 2003 |
| J P Field Consultant Services Ltd | Information & communication | 2003 |
| J.B.H Outsourcing Ltd | Professional & scientific | 2025 |
| J.C.Lewis(Maidenhead)Limited | Transport & logistics | 1963 |
| J.R.Wicks & SON Limited | Real estate | 1948 |
| Jacobs Coffee UK Ltd | Professional & scientific | 2015 |
| Jacobs Douwe Egberts GB Ltd | Wholesale & retail | 1971 |
| Jacobs Douwe Egberts Pro GB Ltd | Manufacturing | 1979 |
| Jacobs Douwe Egberts R&D GB Ltd | Manufacturing | 2015 |
| Jacobs Douwe Egberts UK Pension Trustee Limited | Administrative & support | 2001 |
| Jadecal Signs Limited | Manufacturing | 1983 |
| JAM for Tea Limited | Professional & scientific | 2007 |
| James Allen Limited | Administrative & support | 2004 |
| James Crook & SON (Developments) Limited | | 1973 |
| Jane Myers Associates Limited | Administrative & support | 2004 |
| Jasan Ltd | Professional & scientific | 2003 |
| Jasmin Management Ltd | Professional & scientific | 2004 |
| Jason Bentley Limited | Administrative & support | 2010 |
| Jayborth Properties Limited | Construction | 1972 |
| Jazzy Judes Ltd | Education | 2005 |
| Jbgc Ltd | Public administration | 2022 |
| JC Schofield Ltd. | Information & communication | 2007 |
| Jckm Consulting Limited | Extraterritorial | 2002 |
| JDK IT Limited | Information & communication | 2006 |
| Jentyne Ltd | Wholesale & retail | 2025 |
| Jesco Engineering Company Limited | | 1984 |
| Jethun Solutions Limited | Administrative & support | 2016 |
| Jevents Limited | Transport & logistics | 1995 |
| JI Properties Ltd | Real estate | 2026 |
| Jiffy Products (U.K.) Limited | Wholesale & retail | 1958 |
| Jireh Business Solutions Limited | Information & communication | 2003 |
| JJC Backoffice Solutions Ltd | Financial services | 2025 |
| JLS Haulage Limited | Transport & logistics | 2011 |
| John Cooper Limited | Professional & scientific | 2002 |
| John Cornelius Investments | Real estate | 1964 |
| John Crane UK Limited | Manufacturing | 1923 |
| John George & Sons Limited | Wholesale & retail | 1981 |
| John Griffiths Limited | Wholesale & retail | 2004 |
| John Vetch Properties Ltd | Real estate | 2024 |
| Joinery Plus Limited | Manufacturing | 1993 |
| Jolly Motor U.K. Limited | Transport & logistics | 1989 |
| JON Rosi Management Limited | Real estate | 2001 |
| Jrcs Ltd | Information & communication | 1993 |
| Julie Hogan Ltd | Real estate | 2016 |
| Julise Limited | Extraterritorial | 1984 |
| Jummia Limited | Professional & scientific | 2004 |
| Jurgkor Ltd | Real estate | 2017 |
| JYC Consulting Limited | Professional & scientific | 2001 |
| K & G Software Limited | Information & communication | 1994 |
| K J Rolfe Decorators Limited | Construction | 2002 |
| K.Blyth (Optics) Limited | Extraterritorial | 1949 |
| Kabir Ltd | Construction | 2002 |
| Kadampa Meditation Centre Reading Limited | Education | 1993 |
| Kaizen Partners Accountants & Business Consultants Limited | Professional & scientific | 2006 |
| Karen Kimberley Ltd | Professional & scientific | 2003 |
| Kaushal Jewellers Limited | Wholesale & retail | 2000 |
| KDR Flooring Ltd | Construction | 2017 |
| Keay Consulting Limited | Professional & scientific | 2001 |
| Keeler Limited | Manufacturing | 1946 |
| Keen Young & Co Limited | Professional & scientific | 1994 |
| Keepers Investments Limited | Financial services | 2006 |
| Keith Harris Music Limited | Administrative & support | 1982 |
| Keller Pressure Limited | Manufacturing | 1990 |
| Kemp's Stainless Fasteners Limited | Wholesale & retail | 1980 |
| Kendrick Road (Reading) Management Company Limited | Households | 1984 |
| Kennet Signalling Limited | Professional & scientific | 1998 |
| Kennet Water Limited | Water & waste | 1995 |
| Kennetside Plastics Limited | | 1971 |
| Kenny Road Flats Limited | Households | 2004 |
| Kensington Carter Clinic Limited | Health & social care | 2026 |
| Kept Assets Limited | Financial services | 2018 |
| Key Windows(Uk) Ltd | Professional & scientific | 1997 |
| KFR PRE Press Limited | Manufacturing | 2001 |
| Kgcc Ltd | Information & communication | 2006 |
| Khorys Limited | Real estate | 2005 |
| Khushbu Catering Ltd | Hospitality | 2015 |
| Kimpalm Limited | | 1979 |