| J&J Cottages Limited | Hospitality | 2025 |
| J&J Group Holdings Ltd | Wholesale & retail | 2017 |
| J&J Management Services Limited | Professional & scientific | 2017 |
| J&J Removals Ltd | Administrative & support | 2016 |
| J&L Property (Midlands) Ltd | Real estate | 2016 |
| J&L Shop Ltd | Wholesale & retail | 2023 |
| J&M Capital Group Ltd | Financial services | 2025 |
| J&M Chinese Limited | Hospitality | 2019 |
| J&M Drylining Ltd | Construction | 2023 |
| J&M Swift Logistics Limited | Wholesale & retail | 2024 |
| J&M Tsang Properties Limited | Real estate | 2015 |
| J&N Trans Ltd | Transport & logistics | 2019 |
| J&Nn Properties Ltd | Real estate | 2020 |
| J&O Media Limited | Arts & recreation | 2018 |
| J&P Auto Services Limited | Wholesale & retail | 2020 |
| J&P Autos Limited | Wholesale & retail | 2016 |
| J&P EU Construction Ltd | Construction | 2018 |
| J&S Property Bridgend Ltd | Real estate | 2021 |
| J&S Surfacing Ltd | Construction | 2021 |
| J&S Unit Cars Ltd | Transport & logistics | 2024 |
| J&Sk Enterprise Limited | Hospitality | 2025 |
| J. Carroll Limited | Construction | 2000 |
| J. HAM & Co Limited | Professional & scientific | 2005 |
| J. M. Walden Holdings Limited | Professional & scientific | 2010 |
| J. Plosker & Co. Limited | Extraterritorial | 1941 |
| J. Powell Surveying Services Limited | Professional & scientific | 2023 |
| J. R.'s (Cyncoed) Limited | Other services | 2002 |
| J. Rees Electrical Ltd | Construction | 2021 |
| J. Sherlock & SON Limited | Wholesale & retail | 1998 |
| J. T. Morgan (Butchers) Limited | Wholesale & retail | 2008 |
| J.A. Latham and Sons Property Company Limited | Real estate | 1982 |
| J.A.C. Development Limited | Construction | 1960 |
| J.A.C.S Specialist Limited | Health & social care | 2016 |
| J.A.R. & A.M.R. Commercial Properties Limited | Real estate | 2023 |
| J.A.R. Scaffolding Limited | Construction | 2024 |
| J.C. Building and Electrical Services Limited | Construction | 2002 |
| J.D. Heating and Plumbing and Mechanical Services Limited | Construction | 1998 |
| J.D. Workshop Supplies Limited | Wholesale & retail | 2004 |
| J.D.P.Maintenance Ltd | Construction | 2019 |
| J.E. Edition Limited | Wholesale & retail | 2025 |
| J.Fana Ltd | Construction | 2014 |
| J.Fredricks Landscaping & Building Limited | Construction | 2024 |
| J.G Electrics Limited | Extraterritorial | 2016 |
| J.H.Morgan & Heymann Limited | | 1957 |
| J.Hobley & Co.Limited | Manufacturing | 1934 |
| J.J.L Home Development Ltd | Construction | 2019 |
| J.J.O Tennis World Limited | Arts & recreation | 2008 |
| J.Jones Brickwork Limited | Construction | 2017 |
| J.K Pearson and Sons Ltd | Manufacturing | 2019 |
| J.L Powell Property Lettings Limited | Real estate | 2021 |
| J.L. Sweetman & Sons Ltd | Professional & scientific | 1995 |
| J.L.P. Jean LUC Picard Ltd. | Professional & scientific | 2018 |
| J.M. Walden Limited | Construction | 2005 |
| J.M.W. Construction (Cardiff) Limited | | 1973 |
| J.O'Brien Roofing Ltd | Construction | 2024 |
| J.O.S Consulting Ltd | Information & communication | 2020 |
| J.P. 101 Limited | Hospitality | 2019 |
| J.Price Plumbing & Heating Ltd | Construction | 2025 |
| J.R. Crompton Limited | Manufacturing | 1898 |
| J.R. Smart (Builders) Limited | Construction | 1973 |
| J.R.L Drylining Interiors Ltd | Construction | 2023 |
| J.S.Fry & Sons Limited | Professional & scientific | 1912 |
| J.S.Hussey and Company (Swansea) Limited | Wholesale & retail | 1950 |
| J.Salway & Sons Limited | | 1933 |
| J.Searson Limited | | 1946 |
| J.W. & E. Morris & SON Limited | Manufacturing | 1980 |
| J1H Limited | Professional & scientific | 2025 |
| J2R Ltd | Information & communication | 2021 |
| J3CR Ltd. | Real estate | 2018 |
| J3T Cleaning Services Ltd | Administrative & support | 2026 |
| J4 Asset Management Ltd | Real estate | 2017 |
| J5IA Limited | Financial services | 2025 |
| JA & JL Properties Ltd | Households | 2026 |
| JA & RF Properties Ltd | Real estate | 2023 |
| JA Plumbing & Heating Services Ltd | Construction | 2024 |
| Jaan Café Ltd | Manufacturing | 2025 |
| Jaap Bani Ltd | Information & communication | 2018 |
| JAB Scaffolding Access Ltd | Construction | 2022 |
| JAB Scaffolding Ltd | Construction | 2019 |
| JAB Sound Ltd | Information & communication | 2024 |
| Jabara Limited | Wholesale & retail | 2014 |
| Jabber Products Limited | Wholesale & retail | 2018 |
| Jabir.Mart Ltd | Wholesale & retail | 2025 |
| Jabl Holdings Ltd | Financial services | 2025 |
| JAC Construction Limited | Construction | 2001 |
| JAC Grips Ltd | Information & communication | 2013 |
| JAC J Construction Limited | Construction | 2019 |
| JAC Property Ltd | Real estate | 2025 |
| Jacamo 22 Limited | Wholesale & retail | 2025 |
| Jacaranda Energy Limited | Information & communication | 2018 |
| Jachel Ltd | Real estate | 2023 |
| Jack Evans(Hardware & Ironmongers)Limited | Real estate | 1959 |
| Jack Gabriel Limited | Real estate | 2022 |
| Jack Hackett Ltd | Financial services | 2021 |
| Jack Murphy Bar & Restaurant Ltd | Real estate | 2013 |
| Jack Murphys Uplands Ltd | Hospitality | 2019 |
| Jack Parsley Limited | Construction | 2026 |
| Jackhan Global Supply Limited | Wholesale & retail | 2025 |
| Jackinstant Limited | | 1981 |
| Jacksalt Construction Limited | Construction | 2025 |