| Mccarthy Contractors (Bridgend) Limited | Construction | 1991 |
| Mccarthy Plant (Bridgend) Limited | Transport & logistics | 2018 |
| Mccormelec Limited | Construction | 2022 |
| Mcintosh Construction Surveying Ltd | Construction | 2025 |
| MCK MCR Ltd | Wholesale & retail | 2012 |
| Mckechnie Aerospace DE, LP | | 2017 |
| Mclaren Technology Capital Limited | Wholesale & retail | 2023 |
| MCM United Properties Ltd | Real estate | 2017 |
| Mcsi (South Wales) Limited | Administrative & support | 2025 |
| MD Love Nails Ltd | Other services | 2023 |
| MDA Civils and Structures Engineering Limited | Professional & scientific | 2026 |
| Mdei Ltd | Construction | 2025 |
| MDL Gaming Ltd. | Other services | 2014 |
| MDN Projects Ltd | Professional & scientific | 2023 |
| Meadow Maintenance Ltd | Manufacturing | 2021 |
| Meadows Property Holdings Ltd | Real estate | 2026 |
| Meadowside Property Developments Ltd | Construction | 2025 |
| Meadwell Limited | Construction | 2024 |
| Meah Developments Limited | Real estate | 2024 |
| MEB Solar/Electrical Ltd | Energy | 2022 |
| Mechanical Power Utilities Ltd | Manufacturing | 2023 |
| Medical Venture Ltd | Health & social care | 2014 |
| Medics & Medics Limited | Professional & scientific | 2011 |
| Medrus UK Limited | Professional & scientific | 2000 |
| Meears-White Consulting Limited | Administrative & support | 2018 |
| Megalec Electrical Solutions Ltd | Construction | 2022 |
| Megan Rhys Lucy Transport Limited | Transport & logistics | 2025 |
| Megs Café Limited | Hospitality | 2021 |
| Melanie Eileen Owen Limited | Extraterritorial | 2023 |
| Melanie Lazarus & Co Solicitors Limited | Professional & scientific | 2009 |
| Melgaard Invest Limited | Extraterritorial | 2007 |
| Melpur Ltd | Agriculture & fishing | 2021 |
| Memory IS RAM Limited | Information & communication | 2026 |
| Mental Health Connections Ltd | Agriculture & fishing | 2013 |
| Mental Health Matters Wales Ltd | Health & social care | 2008 |
| Mercy Path Consultancy Limited | Education | 2026 |
| Merecote Limited | Extraterritorial | 2026 |
| Merlin Stores Ltd | Wholesale & retail | 2018 |
| Mesuro Limited | Information & communication | 2008 |
| Metal Goods Limited | Extraterritorial | 2013 |
| Metal Goods Property Limited | Real estate | 2003 |
| Meterfix Limited | Real estate | 2002 |
| MGA Equipment Limited | Wholesale & retail | 2025 |
| MGT Transport Services Limited | Transport & logistics | 2023 |
| MH Sales Consultancy Limited | Wholesale & retail | 2026 |
| Michelle Schutte Limited | Professional & scientific | 2025 |
| Microshade Business Consultants Limited | Information & communication | 1976 |
| Midas IT Solutions Limited | Administrative & support | 2017 |
| Midas Telecom Solutions (SW) Limited | Information & communication | 2012 |
| Midland Securities(Leicester)Limited | Financial services | 1967 |
| Mike Thould Electrical Services Ltd | Construction | 2022 |
| Mili's Restaurant Ltd | Hospitality | 2024 |
| Military Precision Travel & Events Ltd | Administrative & support | 2025 |
| Milleast Limited | Extraterritorial | 2026 |
| Miller Maintenance Limited | Construction | 2026 |
| Miller Transport Consultants Ltd | Transport & logistics | 2022 |
| Millhouse Flats Bridgend Freehold Ltd | Financial services | 2019 |
| Millhouse Veterinary Services Ltd | Professional & scientific | 2014 |
| Millie's Delivery Limited | Hospitality | 2020 |
| Millport Services Limited | Administrative & support | 2013 |
| Milx Heating and Plumbing Limited | Construction | 2023 |
| Minifey Limited | Real estate | 2020 |
| Mining for Diamonds Ltd | Financial services | 2024 |
| Mint Building Services Ltd | Construction | 2020 |
| Mirella Myles Limited | Real estate | 2026 |
| Mirrors and Frames Direct Limited | Manufacturing | 2012 |
| Miss Artisan Limited | Wholesale & retail | 2020 |
| Miss LED Limited | Professional & scientific | 2011 |
| Mission Property Holdings Ltd | Real estate | 2020 |
| Mission PUB Co Ltd | Hospitality | 2018 |
| Mistsolar Group Limited | Financial services | 2013 |
| Mistsolar Holdings Limited | Wholesale & retail | 2017 |
| Mitch & Moore Limited | Wholesale & retail | 2023 |
| Mitchell Klein Ltd | Wholesale & retail | 2022 |
| MIW Consulting Ltd | Professional & scientific | 2018 |
| MIW Property Ltd | Real estate | 2023 |
| MIX Pepper Ltd | Hospitality | 2019 |
| MJ Close Solutions Ltd | Administrative & support | 2024 |
| MJ Construction Solutions Ltd | Construction | 2021 |
| MJ Software Solutions Limited | Information & communication | 2007 |
| MJB Business Consultants Limited | Professional & scientific | 2025 |
| MJE & SON Properties Limited | Real estate | 2026 |
| MJE Homes & SON Limited | Construction | 2026 |
| MJL Plumbing and Bathrooms Ltd | Construction | 2023 |
| MJP Studios Ltd | Information & communication | 2025 |
| ML CAD Ltd | Professional & scientific | 2017 |
| MLP Engineering Ltd | Transport & logistics | 2022 |
| MLR Progress Limited | Professional & scientific | 2002 |
| MMX1 Limited | Real estate | 2016 |
| Mntc Holdings Limited | Financial services | 2015 |
| Mobile Hairdresser Limited | Other services | 2017 |
| Mobile Pipa Testing Ltd | Administrative & support | 2015 |
| Mobile Shore Communications Limited | Information & communication | 2013 |
| Mobilise Cloud Services Limited | Information & communication | 2014 |
| MOD Vent Ltd | Manufacturing | 2019 |
| Modular Clay Products Ltd | Wholesale & retail | 2008 |
| Mohammad Anwar Ltd | Health & social care | 2024 |
| Mohsen Trading Limited | Construction | 2025 |
| Mokan Ltd | Wholesale & retail | 2015 |
| Mollam Wealth Ltd | Financial services | 2024 |