| IST Supplies Limited | Wholesale & retail | 1973 |
| It's Ideal Marketing Ltd | Professional & scientific | 2014 |
| Itenio Limited | Agriculture & fishing | 2015 |
| Iterign Ltd | Professional & scientific | 2011 |
| Itineris Limited | Information & communication | 2001 |
| IV Health Clinic Limited | Health & social care | 2025 |
| Ivac Limited | Extraterritorial | 1972 |
| Izac Ltd | Construction | 2019 |
| I’M in Rehab HUN Ltd | Wholesale & retail | 2025 |
| J & a Earthworks Limited | Construction | 2021 |
| J & M Mangan Limited | Construction | 1992 |
| J & P Transport and Logistics Ltd | Transport & logistics | 2026 |
| J & R Wain Builders Limited | Construction | 2008 |
| J a Delahunty Professional Builders Ltd | Construction | 2012 |
| J Broom Quantity Surveying Services Limited | Professional & scientific | 2012 |
| J C Beale Holdings Limited | Professional & scientific | 2018 |
| J C Event Specialist Ltd | Construction | 2018 |
| J Challis Master Thatchers Ltd | Construction | 2015 |
| J Davies Trading Limited | Transport & logistics | 2022 |
| J Finance Ltd | Financial services | 2004 |
| J J R Properties Limited | Real estate | 1999 |
| J Murray Tiling Limited | Construction | 2021 |
| J S Copsey Limited | Extraterritorial | 1985 |
| J W Gore & Sons LP | | 2010 |
| J Warren Ltd | Agriculture & fishing | 2017 |
| J&Cg Properties LLP | | 2015 |
| J&J Accounting and Taxation Services Ltd | Professional & scientific | 2016 |
| J&S Craftsmen Ltd | Construction | 2019 |
| J&S Shipping & Logistics Ltd | Transport & logistics | 2024 |
| J. J. IT Limited | Information & communication | 2006 |
| J.B.C Groundwork's & Landscaping Ltd | Construction | 2023 |
| J.B.C Haulage Limited | Water & waste | 2025 |
| J.B.C Homes & Developments Ltd Ltd | Construction | 2025 |
| J.I.S.L. Limited | Information & communication | 2020 |
| J.L.R Home Renovations Ltd | Construction | 2025 |
| JAB & JU Ltd | Wholesale & retail | 2024 |
| Jack Channon Racing Limited | Arts & recreation | 2022 |
| Jack Kendrew Ltd | Arts & recreation | 2024 |
| Jack Pinkney Eventing Ltd | Arts & recreation | 2022 |
| Jack Pinkney Livery Ltd | Agriculture & fishing | 2022 |
| Jackson Samuel Limited | Professional & scientific | 2004 |
| Jadecliff Limited | Wholesale & retail | 1982 |
| Jadit Limited | Construction | 2000 |
| Jahojo Limited | Information & communication | 2012 |
| JAI Ford UK Ltd | Arts & recreation | 2026 |
| Jake Warren Limited | Arts & recreation | 2013 |
| JAKE30041961 Limited | Extraterritorial | 2013 |
| Jakki Jones Ltd | Professional & scientific | 2021 |
| Jalan Jalan Limited | Information & communication | 2008 |
| Jalmatech Ltd | Wholesale & retail | 2020 |
| Jamaria PUB Co Ltd | Hospitality | 2025 |
| James Cousins Company | Arts & recreation | 2014 |
| James Cowper Kreston Audit LLP | | 2022 |
| James Cowper Kreston Corporate Finance Limited | Professional & scientific | 2017 |
| James Cowper LLP | | 2008 |
| James Cowper Trustees Limited | Professional & scientific | 2000 |
| James Ferguson Racing Ltd | Arts & recreation | 2019 |
| James O'Kelly Ltd | Professional & scientific | 2017 |
| James Smith Consultancy Limited | Professional & scientific | 2025 |
| Jamie Snowden Racing Limited | Agriculture & fishing | 2021 |
| Jamtech JS Ltd | Professional & scientific | 2019 |
| JAN Kroezen Ltd | Information & communication | 2025 |
| JAP Contract Services Ltd | Construction | 2025 |
| Jascots Wine Limited | Wholesale & retail | 2020 |
| Jasimsaf Berkshire Limited | Information & communication | 2012 |
| Jason Hewetson Limited | Financial services | 2015 |
| JAY Scroggins Services Limited | Construction | 2003 |
| Jazco Limited | Professional & scientific | 2012 |
| JB Groundcare Ltd | Administrative & support | 2019 |
| JC Payroll Services Limited | Professional & scientific | 2003 |
| JC Payroll Solutions Limited | Professional & scientific | 2024 |
| JCH Heating Services (Oxfordshire) Ltd | Construction | 2024 |
| JD Bracey Ltd | Real estate | 2024 |
| JDB Associates Limited | Professional & scientific | 2016 |
| JDM Studios Ltd | Professional & scientific | 2023 |
| JDM Technologies Ltd | Information & communication | 2020 |
| Jedd Consultancy Limited | Information & communication | 2003 |
| Jeffcott Associates Limited | Professional & scientific | 2009 |
| Jenna Burlingham Fine Art Limited | Wholesale & retail | 2010 |
| Jennifer Gillam Photography Limited | Professional & scientific | 2013 |
| Jeremy Smith and Sons (Farriers) Ltd | Agriculture & fishing | 2018 |
| Jesters Inc Ltd | Real estate | 2025 |
| Jewel Training & Development Limited | Professional & scientific | 2008 |
| JF Veterinary Consulting Ltd | Professional & scientific | 2019 |
| JH Settlement Ltd | Financial services | 2020 |
| JIM Hughes Coaches Limited | Transport & logistics | 2003 |
| JIM Nield Countryside Restoration and Management Limited | Administrative & support | 2015 |
| Jinchan Ltd | Information & communication | 2022 |
| Jirc LUX Homes and Rentals Ltd | Real estate | 2025 |
| Jjeh Ltd | Health & social care | 2017 |
| JK Advisory Limited | Professional & scientific | 2016 |
| Jksignalling Ltd | Professional & scientific | 2010 |
| JL Digital Marketing Ltd | Professional & scientific | 2022 |
| JLF Catering Ltd | Hospitality | 2023 |
| JM & LA Property Limited | Real estate | 2023 |
| JMH Heating & Plumbing Services Limited | Construction | 2005 |
| JMI Coaching and Consultancy Ltd | Arts & recreation | 2025 |
| JMI Services Ltd | Professional & scientific | 2025 |
| Jmtm Services Limited | Transport & logistics | 2023 |
| JNB Company Services Ltd | Real estate | 2021 |