| Magic Repair (London) Ltd | Construction | 2021 |
| Magimat Limited | Other services | 1995 |
| Magnet Road Management Company Limited | Households | 2004 |
| Mago Construction Ltd | Construction | 2018 |
| Mahad Trade Mart Limited | Wholesale & retail | 2026 |
| Mahee Limited | Wholesale & retail | 2023 |
| Maidavale Ltd | Other services | 2018 |
| Maidstone Properties Ltd | Real estate | 2017 |
| Mailjet Limited | Professional & scientific | 1985 |
| Mainsiteservify Limited | Information & communication | 2025 |
| Maintenance and Service Matters Limited | Administrative & support | 1996 |
| Maisha Food Ltd | Hospitality | 2021 |
| Maison Shadan Ltd | Wholesale & retail | 2026 |
| Major Properties London Limited | Administrative & support | 1999 |
| Majorbuild Limited | Real estate | 1992 |
| Majors Builder's Merchants (Ilford) Ltd | Wholesale & retail | 1996 |
| MAK UK CA Services London Limited | Professional & scientific | 2023 |
| Makamsales Ltd | Wholesale & retail | 2025 |
| Makdru Consultants Limited | Wholesale & retail | 2006 |
| Make Mine Limited | Hospitality | 2004 |
| Make Surgical Services Limited | Health & social care | 2003 |
| Makin' Steps Agency Ltd. | Arts & recreation | 2003 |
| MAL Brickwork Limited | Construction | 2002 |
| Malachi International Limited | Manufacturing | 2018 |
| Malanest Limited | Wholesale & retail | 1995 |
| Malba Construction Limited | | 1987 |
| Maldon Demolition Limited | Construction | 1999 |
| Maldon Salt Limited | Mining & quarrying | 1926 |
| Malina Patisserie Limited | Hospitality | 2022 |
| Mallard Property Group Limited | Real estate | 2025 |
| Maltacre Limited | | 1984 |
| Malting Paddocks Management Ltd | Households | 2025 |
| Malyk Golt Ltd | Wholesale & retail | 2025 |
| Mamash Ltd | Professional & scientific | 2023 |
| MAN John Limited | Wholesale & retail | 1994 |
| Managing Across Borders Limited | Professional & scientific | 2011 |
| Mandip Limited | Wholesale & retail | 1999 |
| Manifest Store Ltd | Wholesale & retail | 2025 |
| Mann Buck Steels Limited | Administrative & support | 2010 |
| Manor Builders Limited | Construction | 1981 |
| Manor Farm Energy Centre Limited | Energy | 2020 |
| Manor Oaks Limited | Administrative & support | 2003 |
| Manposh Ltd | Wholesale & retail | 2026 |
| Manser & Carter Plastering & Building Limited | Construction | 2007 |
| Mansfield Monk Limited | Professional & scientific | 1998 |
| Manta Trading Ltd | Wholesale & retail | 1999 |
| Mantsbrite Limited | Manufacturing | 1964 |
| Mantsbrite Systems Limited | Information & communication | 1988 |
| Manu Properties Limited | Real estate | 2004 |
| Maplewood Investments Limited | Wholesale & retail | 2003 |
| Maplin Timber Supply Co Limited | Wholesale & retail | 1998 |
| Mapp Security Limited | Professional & scientific | 2003 |
| Mapsyus Info Ltd | Wholesale & retail | 2025 |
| MAR Consultancy Ltd | Health & social care | 2019 |
| Mara TIM Ltd | Construction | 2015 |
| Margaretting (Stratstone) Limited | Professional & scientific | 2025 |
| Mariangelos Logistics Ltd | Other services | 2018 |
| Maries Cross Stitch Limited | Wholesale & retail | 2008 |
| Marinas Restaurants Ltd | Hospitality | 2024 |
| Marine Property Developments Limited | Real estate | 1985 |
| Marisa Nakupenda Milele Ltd | Health & social care | 2023 |
| Marius Repairs Ltd | Manufacturing | 2019 |
| Mark Andrews London Ltd | Professional & scientific | 2014 |
| Mark Conway Limited | Construction | 1997 |
| Mark Dutton Carpentry Limited | Construction | 2007 |
| Mark Harwood Electrical Ltd | Construction | 2015 |
| Mark Polley and Associates Limited | Administrative & support | 2005 |
| Mark Tweddle Construction Services Limited | Professional & scientific | 2024 |
| Mark William Investments Ltd | Real estate | 1972 |
| Marken 1980 Ltd | Construction | 2015 |
| Marketing Kinetics Ltd | Professional & scientific | 1997 |
| Markhor Property Services Ltd | Real estate | 2021 |
| Marko Pollo Antiques Limited | Other services | 2002 |
| Marks Services and Sons Limited | Water & waste | 2005 |
| Marks SW7 Limited | Hospitality | 2025 |
| Marlborough Street SPV1 Ltd | Real estate | 2021 |
| Marline Limited | Construction | 2000 |
| Marpul Limited | Construction | 1997 |
| Marshall Corporation Limited | Construction | 2004 |
| Marshall French Polishing Limited | Construction | 2005 |
| Marshbeck Limited | Wholesale & retail | 2005 |
| Marsom Properties Limited | Real estate | 2023 |
| Martin Dawn (E.A.) Limited | Real estate | 1994 |
| Martin Dawn (Plymouth) Limited | Real estate | 1996 |
| Martin Motor Engineers Limited | Wholesale & retail | 2003 |
| Martins Reach Management Company Limited | Households | 2018 |
| Martlane Limited | Health & social care | 2001 |
| Martyn Howard (Frinton) Limited | Construction | 1972 |
| Marvie Limited | Real estate | 1997 |
| Mary Poppins Day Nursery Limited | Education | 2000 |
| Marzia Traders Ltd | Wholesale & retail | 2025 |
| Masar AL EZZ Group Ltd | Hospitality | 2025 |
| Mashaa Ltd | Wholesale & retail | 2021 |
| Mashtone Capital Limited | Financial services | 2022 |
| Mason & Sons Limited | Wholesale & retail | 1947 |
| Mason Frost Limited | | 1980 |
| Mason Tools | | 1991 |
| Masons (Rentals) Limited | Administrative & support | 1966 |
| Massey Shaw and Marine Vessels Preservation Society Limited | Transport & logistics | 1982 |
| Masterframe Windows Limited | Manufacturing | 1988 |