| Mark Flynn Properties Ltd. | Real estate | 2012 |
| Mark Jermin Management Limited | Administrative & support | 2006 |
| Mark Stone Kitchens Ltd | Manufacturing | 2015 |
| Mark Thomas Properties Ltd | Administrative & support | 2008 |
| Mark Whittaker Engineering Consultancy Services Limited | Professional & scientific | 2013 |
| Market Catalyst Limited | Professional & scientific | 2008 |
| Marks Locksmith Limited | Professional & scientific | 2025 |
| Marlborough Investment Properties Ltd. | Real estate | 2002 |
| Marlyn Land and Developments Limited | Construction | 2019 |
| Marod Enterprises Limited | Wholesale & retail | 2016 |
| Maron Systems Limited | Information & communication | 2003 |
| Marriage Meals Ltd | Hospitality | 2025 |
| Marriejernie Services Limited | Information & communication | 2019 |
| Marsden Real Estate Ltd | Households | 2025 |
| Marshman Holdings Ltd | Financial services | 2024 |
| Marshview Limited | Hospitality | 2023 |
| Mart & Sparks Ltd | Construction | 2023 |
| Martin Associates Limited | Professional & scientific | 2025 |
| Martin Jones Building & Roofing Ltd | Construction | 2013 |
| Martin Solutions Ltd | Education | 2025 |
| Martin Taffetsauffer Building & Civil Engineering Contractors Ltd | Construction | 2006 |
| Martin Taffetsauffer Holdings Limited | Financial services | 2024 |
| Martin Watkins Window Installations Ltd | Construction | 2021 |
| Martinoc Group Ltd | Information & communication | 2025 |
| Martyn Edwards Plumbing Ltd | Construction | 2022 |
| Martyn's Movers Removals & Storage Ltd | Transport & logistics | 2012 |
| Marval Developments Ltd | Construction | 2011 |
| MAS Properties Investments Limited | Real estate | 2024 |
| Masala Llanelli Ltd | Hospitality | 2016 |
| Masava Limited | Health & social care | 2025 |
| Mason & Sons Couriers Ltd | Transport & logistics | 2024 |
| Masse Construction Limited | Construction | 2022 |
| Massilly (UK) Limited | Manufacturing | 1984 |
| Master Catering Supplies Limited | Transport & logistics | 1998 |
| Master School Transport Swansea Limited | Transport & logistics | 2021 |
| Master Truck Driving Services Ltd | Transport & logistics | 2024 |
| Master Waste Solutions Ltd | Water & waste | 2025 |
| Masterchef Gower Limited | Hospitality | 2025 |
| Masterpiece Painting and Decorating Services Ltd | Construction | 2021 |
| Match-It Limited | Information & communication | 1998 |
| Matchday Collectibles Limited | Wholesale & retail | 2025 |
| Matchr Technologies Ltd | Information & communication | 2026 |
| Matera (PVT) Ltd | Manufacturing | 2026 |
| Matha & M Co Ltd | Real estate | 2024 |
| Matsena Productions Limited | Arts & recreation | 2020 |
| Matthew Griffiths Art and Design Limited | Wholesale & retail | 2021 |
| Matthew John Factors Limited | Wholesale & retail | 2020 |
| Matthew Mason Auditing Ltd | Professional & scientific | 2020 |
| Matthew Thomas Landscaping Limited | Construction | 2008 |
| Matthew Tyrrell Ltd | Other services | 2025 |
| Matthews & Co Accountants Limited | Professional & scientific | 2014 |
| Matthews & Co Advisory Limited | Administrative & support | 2018 |
| Matthews Commercial Services Limited | Water & waste | 1997 |
| Matthews Holdings Limited | Financial services | 2018 |
| Maunder Properties Ltd | Construction | 2026 |
| Mauriac Limited | Real estate | 1997 |
| Maurya Properties Ltd | Real estate | 2022 |
| MAV UK Solutions Ltd | Administrative & support | 2024 |
| Mavak Ltd | Wholesale & retail | 2021 |
| Maven Worldwide Limited | Real estate | 2016 |
| Mavento Ltd | Wholesale & retail | 2025 |
| MAX Hearing Limited | Health & social care | 2012 |
| MAX TEW Properties Limited | Real estate | 2017 |
| Maxess Limited | Construction | 1992 |
| Maxim Deli Limited | Hospitality | 2022 |
| Maximize Soccer Ltd | Education | 2026 |
| Maximus Italian Ltd | Hospitality | 2025 |
| Maximus Property Investors Ltd | Construction | 2014 |
| Maxmac Ltd | Hospitality | 2024 |
| Maxmill Ltd | Real estate | 2024 |
| Maxonique Brow and Nail Bar Ltd | Other services | 2019 |
| Maxx Swansea Limited | Wholesale & retail | 2014 |
| MAY Rose & Cameron Ltd | Professional & scientific | 2018 |
| Mayals Investments Limited | Real estate | 2023 |
| Mayandjules Properties Ltd | Real estate | 2025 |
| Mayberry Jones Limited | Real estate | 1946 |
| Maybery Financial Services Ltd | Financial services | 2023 |
| Maybright Cleaning Limited | Administrative & support | 2008 |
| Mayday Cleaning Ltd | Administrative & support | 2007 |
| Mayday Coffee Co Ltd | Hospitality | 2023 |
| Mayhill Stores Limited | Wholesale & retail | 2022 |
| Maytree Cottages Limited | Real estate | 2025 |
| Mazii Kebab Limited | Hospitality | 2018 |
| Mazvita Ltd | Health & social care | 2019 |
| MB Audio Visual Limited | Wholesale & retail | 2003 |
| MB Companies Limited | Construction | 2026 |
| MB Film Production Limited | Arts & recreation | 2021 |
| MB Freight & Logistics (UK) Limited | Transport & logistics | 2021 |
| MB Music Productions Ltd | Information & communication | 2024 |
| MB Plant Training Limited | Administrative & support | 2004 |
| MBC Commercial Ltd | Professional & scientific | 2018 |
| MBC UK Holdings Limited | Construction | 2023 |
| MBE Safety Consultants Limited | Professional & scientific | 2018 |
| MBP Radiology Services Ltd | Health & social care | 2018 |
| MC Cowshed Limited | Hospitality | 2024 |
| MC Doner Swansea Limited | Hospitality | 2022 |
| MC Law Ltd | Extraterritorial | 2014 |
| MC Ridgewell Limited | Health & social care | 2005 |
| MC500 Limited | Manufacturing | 2004 |
| Mcalinden Surgical Ltd | Health & social care | 2025 |