| Isocount UK Limited | Manufacturing | 2012 |
| ITG Groundworks Ltd | Construction | 2026 |
| IWM Financial Planning Ltd | Professional & scientific | 2026 |
| IWM Wealth Management Ltd | Financial services | 2017 |
| J & a K Consultancy Limited | Professional & scientific | 2025 |
| J & K Properties (Gwent) Ltd | Real estate | 2024 |
| J Gareth Morgan & Co (2011) LLP | | 2011 |
| J Knott Consultancy Ltd | Professional & scientific | 2025 |
| J TEK Properties Limited | Real estate | 2026 |
| J W Homes Ltd | Real estate | 2011 |
| J-Apex Properties Ltd | Real estate | 2025 |
| J. & W. Scaffolding (Ponthir) Limited | | 1984 |
| J.A.K.S Developments Ltd | Real estate | 2021 |
| J.A.K.S National Ltd | Financial services | 2021 |
| J.R. Gilbert Engineering Limited | Manufacturing | 1983 |
| J.S Properties (JSP) Ltd | Real estate | 2022 |
| J.T.C Motorhomes Ltd | Wholesale & retail | 2021 |
| JAC (Wales) Limited | Wholesale & retail | 2002 |
| JAC Jones Construction Limited | Construction | 2013 |
| Jacob’S Fish & Chips Ltd | Hospitality | 2022 |
| Jade Garden Ltd | Hospitality | 2003 |
| Jalwood Property Co. Limited | Households | 1975 |
| Jansan Wear Ltd | Wholesale & retail | 2013 |
| JAS & Co Properties Limited | Real estate | 2026 |
| Jason and Luke Property Solutions Ltd | Real estate | 2017 |
| JAY Jones Property Ltd | Extraterritorial | 2025 |
| Jayci Plant Ltd | Manufacturing | 2022 |
| JB Properties (Gwent) Limited | Real estate | 2017 |
| JB Property Solutions Ltd | Real estate | 2025 |
| JCS Businesses Limited | Hospitality | 2014 |
| JD Carpentry SW Ltd | Construction | 2025 |
| Jdeec Ltd | Transport & logistics | 2022 |
| JE Rendering and Renovation Limited | Construction | 2023 |
| JEJ Systems Limited | Information & communication | 2024 |
| Jelly Totz Childcare Limited | Health & social care | 2013 |
| Jenkins Family Investments Limited | Financial services | 2016 |
| Jeyasutha Property Ltd | Real estate | 2018 |
| JIM Davies Civil Engineering Limited | Construction | 1976 |
| JIM Davies Holdings Limited | Professional & scientific | 2002 |
| JJR Property Services Limited | Professional & scientific | 2020 |
| JLJ Property Holdings Ltd | Real estate | 2020 |
| JLT Plumbing & Heating Ltd | Construction | 2015 |
| JN Tools and Fixings Limited | Wholesale & retail | 2009 |
| Joanna Homes Property Management Limited | Households | 2019 |
| Joanna’S Cleaning Service Ltd | Administrative & support | 2026 |
| John Dunn Brickwork Contractors Ltd | Construction | 2009 |
| Johns OIL and GAS Limited | Construction | 2022 |
| Johnson James Properties Limited | Real estate | 2022 |
| Johnson White Financial Services Ltd | Professional & scientific | 2013 |
| JOK Transport Ltd | Transport & logistics | 2022 |
| Jollys Carpets Ltd | Wholesale & retail | 2019 |
| JON Bishop Construction Ltd | Construction | 2011 |
| Jones & SON Property Ltd | Real estate | 2025 |
| Jones Civils Group Limited | Construction | 2024 |
| Jpaconsultancy Limited | Professional & scientific | 2016 |
| Jpeg Trading Ltd | Construction | 2020 |
| JPL Solutions Ltd | Wholesale & retail | 2024 |
| JR Everything Autos Ltd | Wholesale & retail | 2026 |
| JR Welding Services Limited | Professional & scientific | 2018 |
| JRB Mortgages Ltd | Financial services | 2022 |
| Jtpd Limited | Health & social care | 2018 |
| Just Love Food Company Holdings Limited | Manufacturing | 2020 |
| Just Love Food Company Ltd | Manufacturing | 2009 |
| JV Tile & Style Ltd | Construction | 2021 |
| K & S Family Leisure Services Ltd | Health & social care | 2018 |
| K and M Upholstery Services Limited | Manufacturing | 2014 |
| K&M Automotive Ltd | Wholesale & retail | 2024 |
| K.L.O Electrical Ltd | Construction | 2025 |
| Kairlo Limited | Health & social care | 2021 |
| Kaizen Building Ltd | Construction | 2025 |
| Kamfran Limited | Health & social care | 2024 |
| Karl Foster Associates Limited | Professional & scientific | 2017 |
| KD Systems (KDS) Ltd | Manufacturing | 2018 |
| Kingston Pharmaceutical Services Limited | Professional & scientific | 2025 |
| Kippax Signature Homes Limited | Construction | 2024 |
| Kira Wild Coaching Ltd | Arts & recreation | 2023 |
| Kusten Vorland Ltd | Construction | 2009 |
| Kyle Martin - Health Psychology Consulting Limited | Health & social care | 2024 |
| L R Product Design Limited | Manufacturing | 2008 |
| L T Industrial Cladding Ltd | Construction | 2019 |
| LAD Technologies Limited | Information & communication | 2018 |
| Laines Construction Limited | Construction | 2019 |
| Lakestone Properties Limited | Real estate | 2022 |
| LB Bricklaying Ltd | Construction | 2023 |
| LCS Engineering Ltd | Construction | 2009 |
| LD Taste Limited | Hospitality | 2021 |
| Leadingedge Health Assessments Ltd | Health & social care | 2020 |
| Lean & Mean Finance Limited | Professional & scientific | 2020 |
| Leanne the Celebrant Ltd | Other services | 2025 |
| Lectogic Limited | Manufacturing | 2010 |
| Leecom Ltd | Construction | 2024 |
| Lefia Ltd | Wholesale & retail | 2023 |
| Lemr Lettings Ltd | Real estate | 2026 |
| Lenex Rendering Ltd | Construction | 2015 |
| Lenex Rendering Supplies Ltd | Construction | 2015 |
| LET Lewis Limited | Construction | 2005 |
| Levante Ltd | Real estate | 2019 |
| LEW Asset & Property Ltd | Real estate | 2023 |
| LGD Property Investments Limited | Real estate | 2024 |
| LGR Services Limited | Manufacturing | 2009 |