| J. John Chuan Ltd. | Information & communication | 2002 |
| J.A. Fisher Cars Limited | Transport & logistics | 1959 |
| J.A. Steel & SON Limited | Transport & logistics | 1999 |
| J.B. Surveying Limited | Professional & scientific | 2003 |
| J.H. (Bermondsey) Limited | | 1933 |
| J.H.S.Properties(London)Limited | Real estate | 1972 |
| J.K. Tipper Services Limited | Transport & logistics | 1997 |
| J.L. Croft (Teddington) Limited | Construction | 1984 |
| J.L. Lyons & Company Limited | Construction | 1980 |
| J.P. Properties Overseas Ltd | Households | 2022 |
| J2Y (Holdings) Ltd | Financial services | 2024 |
| J2Y Ltd | Real estate | 2024 |
| JAC Hyman Ltd | Administrative & support | 2008 |
| Jack & Jill School Limited | Education | 1978 |
| Jackamoor Builders Supplies Limited | | 1981 |
| JAD Realty Limited | Real estate | 2025 |
| Jade Capital Ltd | Real estate | 2019 |
| Jadiwana Limited | Hospitality | 1977 |
| Jafs Spices Limited | Hospitality | 2019 |
| Jafvans Limited | Transport & logistics | 1997 |
| Jagged Consulting Limited | Information & communication | 2007 |
| JAI Shri RAM Barbershop Ltd | Other services | 2025 |
| Jaiden Limited | Transport & logistics | 2005 |
| Jainil Ltd | Water & waste | 1995 |
| Jaks Property Investments Limited | Real estate | 1993 |
| Jala Krupa Ltd | Wholesale & retail | 2004 |
| Jalay Enterprises Limited | Real estate | 1971 |
| Jalsa (UK) Ltd | Administrative & support | 2003 |
| JAM Imaging Limited | Professional & scientific | 2009 |
| Jamal & Associates Limited | Real estate | 1988 |
| James Brooke & Sons Limited | | 1925 |
| James Giltinane | | 1998 |
| James. Polean | | 1998 |
| Jane Aspinall Designs Limited | Other services | 1980 |
| Jane Spicer Costumes Limited | Professional & scientific | 2023 |
| Japan Cars UK Ltd | Wholesale & retail | 2005 |
| Japotheca Ltd | Wholesale & retail | 2004 |
| Jaques Resources Limited | Information & communication | 2005 |
| Jarak Limited | Information & communication | 1999 |
| Jariv Management Limited | Construction | 2022 |
| JAS Fashion Ltd | Wholesale & retail | 2014 |
| Jash Property Limited | Real estate | 2006 |
| Jaxward Limited | Information & communication | 1980 |
| JAY KAY & Y Ltd | Hospitality | 2005 |
| Jaysam Contractors Limited | Construction | 1986 |
| JB Electrical Services Group Limited | Construction | 2025 |
| JBK Trustees Limited | Professional & scientific | 2021 |
| Jcdecaux Limited | Professional & scientific | 1964 |
| Jcdecaux UK Limited | Professional & scientific | 1982 |
| Jcdecaux United Limited | Professional & scientific | 1964 |
| JCL Installations Limited | Construction | 2006 |
| JDN Accountants Ltd | Professional & scientific | 2007 |
| JDS Dining Ltd | Hospitality | 2025 |
| JDW Media Ltd | Information & communication | 2015 |
| JE Groundwork & Landscape Ltd | Construction | 2025 |
| Jeffery-Crocker Limited | Information & communication | 1996 |
| Jefferys Group Ltd | Real estate | 2001 |
| Jennor Holdings Limited | Financial services | 2016 |
| Jetgame Limited | Information & communication | 1992 |
| Jewish Heritage Foundation | Professional & scientific | 1999 |
| JF Ferreira Limited | Construction | 2024 |
| Jg-Jm Services Limited | Other services | 2007 |
| JGM Accounting Services Limited | Financial services | 2002 |
| Jigsaw Mechanical Services Limited | Construction | 2007 |
| Jink Enterprises Limited | Extraterritorial | 2007 |
| Jiram (London) Limited | Financial services | 2022 |
| Jivraj Food & Wine Limited | Wholesale & retail | 2025 |
| JJC Plumbers Ltd | Construction | 2018 |
| JJT UK Services Limited | Administrative & support | 2024 |
| JK Dental Design (Laboratory) Ltd | Health & social care | 2003 |
| JMB Group of Companies Ltd | Financial services | 2020 |
| JMG Property Services Limited | Real estate | 2004 |
| JMR Lettings Ltd | Real estate | 2023 |
| JOE Wenborne Productions Limited | Arts & recreation | 2006 |
| John Cleary | | 1998 |
| John Cotter Limited | Construction | 1988 |
| John Garnham Consultancy Limited | Professional & scientific | 2007 |
| John James. Byrne | | 1998 |
| John Nevin Limited | | 1965 |
| John Ogle Ltd. | Professional & scientific | 2001 |
| John Williams (Marketing) Limited | | 1989 |
| Jojocadelle Designs Limited | Arts & recreation | 2026 |
| Jolimarc Limited | Professional & scientific | 1983 |
| Jomax Limited | | 1984 |
| JON Oliver Studio Limited | Arts & recreation | 2013 |
| Jonathan Weatherley Limited | Arts & recreation | 2015 |
| Jordasic (UK) Limited | Professional & scientific | 1998 |
| Joseph Sterling Limited | Wholesale & retail | 2003 |
| Joseph Surgical Services Limited | Health & social care | 2017 |
| Josh Weston FX Limited | Information & communication | 2026 |
| JPF Limited | Administrative & support | 2007 |
| JR Asset Holdings Ltd | Real estate | 2025 |
| JR of Ashford Limited | Wholesale & retail | 2001 |
| JR Renewables Limited | Construction | 2021 |
| JR Solo Limited | Financial services | 1999 |
| Jradia Ltd | Health & social care | 2024 |
| JRD Management Limited | Administrative & support | 2012 |
| JRP Enterprises Limited | Professional & scientific | 2004 |
| JSD Foods Ltd | Hospitality | 2018 |
| JSG Services Ltd | Information & communication | 2002 |