| IOK Digital Limited | Manufacturing | 2020 |
| Irins Kitchen Ltd | Manufacturing | 2025 |
| Island Farm Donkey Sanctuary | Arts & recreation | 2007 |
| Itil Limited | Information & communication | 2004 |
| Ixia PA | Information & communication | 2003 |
| J and G Building and Plastering LLP | | 2023 |
| J and G Properties Limited | Construction | 1932 |
| J Baker Consulting Limited | Professional & scientific | 2006 |
| J C Brant Limited | Other services | 2022 |
| J L Bosley Limited | Real estate | 1961 |
| J S Auto Services Limited | Wholesale & retail | 2015 |
| J.M.L.E. Solutions Ltd | Professional & scientific | 2012 |
| J.P Executive Services Ltd | Information & communication | 2025 |
| Jaho UK Trading Ltd | Wholesale & retail | 2025 |
| James Mercel Ltd | Manufacturing | 2026 |
| Jane Fryatt Ltd | Administrative & support | 2017 |
| JAR Enterprise Limited | Real estate | 2018 |
| Jare Property Developments Ltd | Construction | 2024 |
| JB SIV Consulting Ltd | Professional & scientific | 2019 |
| Jctr Ltd | Professional & scientific | 2016 |
| JDM Construction & Civils Ltd | Construction | 2024 |
| JDS Joinery Limited | Manufacturing | 2024 |
| JEM Chimney Sweep Limited | Administrative & support | 2003 |
| Jenni Lewis Ltd | Information & communication | 2011 |
| Jessecco Prosecco Ltd | Hospitality | 2023 |
| JFH Fine Art Consultancy Limited | Professional & scientific | 2013 |
| JG42 Ltd | Health & social care | 2024 |
| JIM Bean Coffee Company Ltd | Hospitality | 2015 |
| JIM Pink (Wallingford) Limited | Wholesale & retail | 1976 |
| JJ Cuzen Building Contractors Limited | Construction | 2013 |
| Jjkay Limited | Financial services | 2025 |
| JK Branding & Communications Limited | Information & communication | 2017 |
| JLS Global Consulting Ltd | Professional & scientific | 2009 |
| Johnstone Place Residents Association Limited | Households | 1993 |
| Jonas Collections and Recovery Limited | Information & communication | 2021 |
| Jonas Computing (UK) Limited | Information & communication | 2004 |
| Jonas Event Technology Limited | Administrative & support | 2011 |
| Jonas Lifestyle Limited | Information & communication | 2001 |
| Jonas Metals Software Limited | Information & communication | 1991 |
| Jonas Sports Limited | Information & communication | 2008 |
| Jones-Metterklume Limited | Information & communication | 2019 |
| Jonty Hearnden Limited | Wholesale & retail | 2001 |
| Jorr 37 Limited | Wholesale & retail | 2015 |
| Josh&Jef Ltd | Real estate | 2024 |
| Journey's END Limited | Construction | 2001 |
| JRC Carpentry and Building Limited | Construction | 2025 |
| Jspark Limited | Professional & scientific | 2010 |
| Jspr LLP | | 2022 |
| Jtec UK Limited | Information & communication | 2004 |
| JTK Consulting Ltd | Professional & scientific | 2026 |
| Julia James PIC Limited | Financial services | 2021 |
| Julie Maunder Interiors Ltd | Professional & scientific | 2008 |
| Just One in a Crowd Limited | Information & communication | 2011 |
| Just Trading of Wallingford Limited | Wholesale & retail | 2005 |
| JW Architects Ltd | Professional & scientific | 2008 |
| JXG Services Ltd | Professional & scientific | 2015 |
| K G S Stores Ltd | Wholesale & retail | 2013 |
| K S Food Equipment Limited | Other services | 2011 |
| K&E Pristine Cleaning Limited | Administrative & support | 2026 |
| KDB Property Development Ltd | Real estate | 2023 |
| Keep Developments Limited | Real estate | 2015 |
| Kennet Studios Ltd | Real estate | 2017 |
| Kennylands Investments Limited | Real estate | 1963 |
| Kentech Instruments Limited | Manufacturing | 1983 |
| Keylogik Ltd | Wholesale & retail | 2026 |
| KID Chat Speech and Language Therapy Ltd | Health & social care | 2014 |
| Kilkenny Turner Ltd | Real estate | 2025 |
| Kilmington Properties Ltd | Real estate | 2015 |
| King's Acre Residents Management (1991) Co Ltd | Households | 1990 |
| Kingsford House RTM Company Limited | Households | 2026 |
| Kingwood Park Management Company Limited | Households | 1992 |
| Kirkman Joinery Ltd | Manufacturing | 2012 |
| Kiseki Consulting Ltd | Professional & scientific | 2021 |
| Kite Real Estate Limited | Real estate | 2018 |
| KJG IT Services Limited | Information & communication | 2014 |
| Kkdc Design House Ltd | Arts & recreation | 2014 |
| Klein Consultancy and Advisory Services Limited | Administrative & support | 2022 |
| Klein Solicitors Limited | Professional & scientific | 2007 |
| KLH Hideaways Ltd | Administrative & support | 2021 |
| Klimex Ltd | Transport & logistics | 2023 |
| KLN Consulting Limited | Other services | 2023 |
| KLO Partners Ltd | Professional & scientific | 2024 |
| KMH Hospitality Solutions Ltd | Hospitality | 2019 |
| KMS Power Consulting Ltd | Professional & scientific | 2021 |
| Knightsbridge Media Ltd | Other services | 2025 |
| Knofell Design Ltd | Manufacturing | 2004 |
| Kovara Holdings Limited | Construction | 2026 |
| Kovara Projects Limited | Construction | 2004 |
| KP Wallingford Ltd | Wholesale & retail | 2022 |
| Kpcm Ltd | Professional & scientific | 2018 |
| Kraaivlei Konsulting Ltd | Mining & quarrying | 2021 |
| KW Consultants Ltd | Health & social care | 2021 |
| Laguna Medical Ltd | Health & social care | 2015 |
| Lakeside Energy From Waste Holdings Limited | Professional & scientific | 2005 |
| Lakeside Energy From Waste Limited | Water & waste | 1999 |
| Lamert Properties Limited | Real estate | 2024 |
| Larkmead Holdings Limited | Financial services | 2021 |
| Larkmead Practice LLP | | 2012 |
| Larkmead Veterinary Group Limited | Professional & scientific | 2012 |
| Laudanum House Limited | Wholesale & retail | 2024 |