| M Squared Consultancy Life Sciences Limited | Professional & scientific | 2025 |
| M Summerhayes Services Ltd | Professional & scientific | 2025 |
| M W HOY Electrical Limited | Construction | 2017 |
| M&D Origins Ltd | Wholesale & retail | 2023 |
| M. Day Building Contractors Limited | Construction | 2012 |
| M. GAS Limited | Construction | 2007 |
| M. J. Steeden Limited | Health & social care | 2016 |
| M.G.Plumbing Co.Limited | | 1969 |
| M.J. Warner (Holdings) Limited | Professional & scientific | 1978 |
| M.Matheou Ltd | Construction | 2016 |
| M.W.T. Harris & Co. Limited | | 1982 |
| M4K Ltd | Professional & scientific | 2020 |
| MA & SJ Odell Limited | Administrative & support | 2005 |
| Maben Installations Ltd | Construction | 2018 |
| MAC and Sons Herts Ltd | Other services | 2019 |
| Maccon Properties Limited | Real estate | 2016 |
| Mach Technology Ltd | Information & communication | 2018 |
| Maclock Engineering Ltd | Professional & scientific | 2012 |
| Macnaughtan Holdings Ltd | Financial services | 2025 |
| Macollison Limited | Real estate | 2024 |
| Macrame Limited | Real estate | 2008 |
| Macsa ID UK Ltd | Manufacturing | 2012 |
| MAD Professor Limited | Arts & recreation | 2007 |
| Maddox Groundworks Ltd | Construction | 2026 |
| Madverse Ltd | Information & communication | 2024 |
| Magenta Estates Limited | Construction | 2020 |
| Magic Productions Ltd | Information & communication | 2019 |
| Magic WOK Hitchin Ltd | Hospitality | 2021 |
| Magnolia Inn Limited | Hospitality | 2019 |
| Maicounts Consultancy Limited | Financial services | 2014 |
| Maiden Electrical Services Ltd | Construction | 2025 |
| Maison Nequi Ltd | Wholesale & retail | 2026 |
| MAKKA1 Limited | Real estate | 2023 |
| Mallion Consulting Limited | Professional & scientific | 2013 |
| Maltings Orchard Pirton Management Limited | Households | 1995 |
| Mamababybliss Ltd | Wholesale & retail | 2007 |
| Mamababybliss Social Ventures CIC | Health & social care | 2016 |
| Mamababyplay Ltd | Arts & recreation | 2020 |
| MAN Overboard Ltd | Professional & scientific | 2019 |
| Mana 433 Limited | Professional & scientific | 2014 |
| Managing London Ltd | Real estate | 2013 |
| Mandy Cuttill Limited | Arts & recreation | 2014 |
| Mangan Homes Limited | Real estate | 2022 |
| Mangat Investments Limited | Real estate | 2016 |
| Manifest International Ltd | Wholesale & retail | 2022 |
| Mannat M Ltd | Wholesale & retail | 2024 |
| Manor House Farm Limited | Real estate | 2007 |
| Mantelpiece PR Ltd | Professional & scientific | 2009 |
| Manuals Drapes Limited | Manufacturing | 2003 |
| Manufacturing Support and Design Limited | Manufacturing | 2000 |
| Manyau Social Care Ltd | Health & social care | 2018 |
| Maon Capital Group Holdings Ltd | Financial services | 2024 |
| Maon Financial Associates Ltd | Financial services | 2024 |
| Maon Professional Services Ltd | Professional & scientific | 2024 |
| Maple Marketing (U.K.) Ltd. | Information & communication | 1993 |
| Marasis Holdings Limited | Professional & scientific | 2015 |
| Marble Arch Consultancy Limited | Information & communication | 2013 |
| Marc Brittain Barrister Limited | Professional & scientific | 2020 |
| Marc COX Ltd | Real estate | 2022 |
| Marchwood Group Limited | Financial services | 2024 |
| Marchwood Recruitment Limited | Administrative & support | 2024 |
| Marcin's Construction Ltd | Construction | 2024 |
| Marco FM Ltd | Manufacturing | 2021 |
| Marco Technologies Limited | Manufacturing | 2019 |
| Marcus Davey & Co Ltd | Information & communication | 2024 |
| Mardan Building Services Limited | Construction | 2015 |
| Mardan Development (Beds) Limited | Construction | 2022 |
| Mardoc Properties Development LLP | | 2017 |
| Marerimedia Ltd | Professional & scientific | 2014 |
| Maressa Properties Limited | Real estate | 2009 |
| Marex Properties Limited | Real estate | 1999 |
| Margaret House (Hertfordshire) Limited | Health & social care | 2025 |
| Margaret House Holdings Limited | Financial services | 2025 |
| Margaret Long Consulting Limited | Administrative & support | 2026 |
| Mariano Hairbar Ltd | Other services | 2023 |
| Marie Polley Consultancy Ltd | Professional & scientific | 2025 |
| Marinations Kitchen Limited | Hospitality | 2025 |
| Marine Tax Consultants Limited | Professional & scientific | 2023 |
| Mariposa of London Limited | Extraterritorial | 2006 |
| Mark and Sons Mobile Mechanics Ltd | Wholesale & retail | 2024 |
| Mark Burrows Ltd | Professional & scientific | 2004 |
| Mark Higginson (MH) Care Ltd | Health & social care | 2015 |
| Mark Parish Limited | Wholesale & retail | 2011 |
| Mark Richard Penny Limited | Other services | 2014 |
| Mark Sullivan Carpentry Ltd | Construction | 2017 |
| Marketing for Hospitality Limited | Hospitality | 2016 |
| Marketmaker Media Limited | Information & communication | 2009 |
| MARKS84 Ltd | Real estate | 2021 |
| Marloch Properties Ltd | Real estate | 2024 |
| Marmet Limited | Extraterritorial | 1990 |
| Martha Lund Limited | Information & communication | 2024 |
| Martin Beale Fabrications & Metalwork Limited | Manufacturing | 2001 |
| Martin Sheds Ltd | Manufacturing | 2014 |
| Martins Wood Management Company Limited | Households | 1981 |
| Martyn Foster Ltd | Wholesale & retail | 2025 |
| Mason Navarro Pledge Holdings Limited | Financial services | 2024 |
| Mason Navarro Pledge Limited | Professional & scientific | 1999 |
| Mason Pharma Services Ltd. | Professional & scientific | 2015 |
| Masons BTL Ltd | Real estate | 2020 |
| Masons Prestige Limited | Wholesale & retail | 2014 |