| Janacom Limited | Information & communication | 2008 |
| Janet D Jenkins Limited | Professional & scientific | 2016 |
| Janr Global Ltd | Wholesale & retail | 2014 |
| Janus Medical Reporting Limited | Health & social care | 2004 |
| Jaoyeh Trading Ltd | Wholesale & retail | 2012 |
| JAP Business Solutions Limited | Administrative & support | 2005 |
| JAQ Holdings Limited | Financial services | 2021 |
| JAR Building Services Ltd | Construction | 2019 |
| Jardim & Santos Ltd | Hospitality | 2025 |
| Jarekans Ltd | Wholesale & retail | 2023 |
| Jares Group Ltd | Administrative & support | 2023 |
| Jarrington Properties Limited | Real estate | 2013 |
| Jarv's Energy Ltd | Real estate | 2020 |
| Jarv's Group Limited | Administrative & support | 2016 |
| Jarv's Holdings Limited | Real estate | 2019 |
| Jarv's Homes Ltd | Construction | 2016 |
| Jarv's Leisure Limited | Hospitality | 2016 |
| Jarvis Strong Productions Limited | Information & communication | 2023 |
| JARVIS2 Ltd | Professional & scientific | 2007 |
| Jasara Property Limited | Construction | 2021 |
| Jasiva Ltd | Wholesale & retail | 2024 |
| Jasko Group Ltd | Real estate | 2019 |
| Jasmin Howarth Homes Ltd | Real estate | 2024 |
| Jasmine Thai Catering Ltd | Hospitality | 2010 |
| Jason Banbury Limited | Professional & scientific | 2000 |
| Jason's Tyre Services Ltd | Wholesale & retail | 2020 |
| Jast Ltd | Real estate | 2024 |
| Javalin Property Developments Ltd | Real estate | 2022 |
| JAW Software Limited | Information & communication | 2002 |
| Jawos 10 Ltd | Financial services | 2020 |
| Jaxit Limited | Information & communication | 2008 |
| Jaxx Autos Limited | Wholesale & retail | 2020 |
| Jaxx Bay Limited | Construction | 1999 |
| Jaxx Harbour Holdings Limited | Professional & scientific | 2010 |
| Jaxx Harbour Limited | Construction | 2010 |
| Jaxx Padel Limited | Arts & recreation | 2024 |
| Jaxx Tower Limited | Real estate | 2016 |
| JAY Harris Boxing & Fitness Limited | Arts & recreation | 2025 |
| JAY Medicaments Ltd | Health & social care | 2022 |
| JAY Nails Ltd | Other services | 2025 |
| JAY Partridge Coaching Ltd | Arts & recreation | 2025 |
| Jay's Barbering Ltd | Other services | 2015 |
| Jaycock (Trading) Limited | Administrative & support | 2006 |
| Jayfairy Ltd | Other services | 2025 |
| Jaygee Limited | Professional & scientific | 2024 |
| Jaylex Consulting Limited | Administrative & support | 2015 |
| Jaylex Properties Ltd | Real estate | 2016 |
| Jayne's Boutique Wales Limited | Wholesale & retail | 2021 |
| Jays Portfolio Limited | Real estate | 2020 |
| JAZ & Veer Limited | Wholesale & retail | 2019 |
| Jazz R Properties Limited | Real estate | 2015 |
| JB Coaching (South Wales) Ltd | Arts & recreation | 2023 |
| JB Heating (UK) Limited | Construction | 2008 |
| JBB Solutions Ltd | Information & communication | 2023 |
| JBS Stores Ltd | Wholesale & retail | 2025 |
| Jbskitchen Ltd | Extraterritorial | 2024 |
| JC Irvine Limited | Construction | 2007 |
| JC Levels Limited | Construction | 2026 |
| JC Property Inspections Ltd | Real estate | 2019 |
| Jc.Flights Ltd | Transport & logistics | 2022 |
| JCB1 Dental Limited | Health & social care | 2019 |
| Jcbc Ltd | Administrative & support | 2023 |
| JCD Property Investors Ltd | Real estate | 2023 |
| JCG Surveys Ltd | Other services | 2017 |
| JCH Property Group Ltd | Real estate | 2025 |
| JCK Industrial Limited | Wholesale & retail | 2016 |
| JCM Accounts Limited | Professional & scientific | 2021 |
| JCP Holdings & Investments Limited | Financial services | 2023 |
| JCP Solicitors Limited | Professional & scientific | 2014 |
| Jcpj Ltd | Transport & logistics | 2024 |
| JCR & AJR Holdings Ltd | Construction | 2025 |
| JCR Rail Limited | Construction | 2022 |
| JCT Performance Coaching Limited | Education | 2024 |
| JCT48 Storage Limited | Administrative & support | 2017 |
| JD MFG Ltd | Wholesale & retail | 2020 |
| JD Swansea Limited | Arts & recreation | 2023 |
| JD Telecommunication Ltd | Construction | 2026 |
| Jd&Co Limited | Real estate | 2023 |
| Jdautomotiveco CYF | Administrative & support | 2026 |
| JDB Car, Bike and Van Services Ltd | Wholesale & retail | 2025 |
| JDC Recovery Ltd | Transport & logistics | 2025 |
| JDD Builders Limited | Construction | 2016 |
| JDF Developments Limited | Professional & scientific | 2007 |
| JDH Consultancy Limited | Real estate | 2021 |
| JDH Pubs (Lamb & Flag) Ltd | Hospitality | 2023 |
| JDK Cleaning Limited | Administrative & support | 2012 |
| JDK Travel Limited | Transport & logistics | 2024 |
| JDP Civils Ltd | Construction | 2026 |
| JDR Associates Limited | Health & social care | 2012 |
| JDS Couriers Ltd | Transport & logistics | 2021 |
| JDS Systems Ltd | Administrative & support | 2017 |
| JDS Team Limited | Hospitality | 2020 |
| JE Digital Services Ltd | Information & communication | 2026 |
| Jeannette Mclellan Limited | Professional & scientific | 2003 |
| Jecom Limited | Wholesale & retail | 2025 |
| Jeddyshaw Limited | Information & communication | 2023 |
| Jeff Morgan Taxis Ltd | Transport & logistics | 2003 |
| Jeffriesr Ltd | Professional & scientific | 2021 |
| JEH Swansea Limited | Real estate | 2023 |
| JEJ Electrical Services Limited | Construction | 2011 |