| Maintenance Contract Solutions Limited | Administrative & support | 2026 |
| Maisiemay’S Limited | Other services | 2023 |
| Majid Barbers Limited | Other services | 2024 |
| Make a Difference Fitness PT Studio Limited | Wholesale & retail | 2017 |
| Make Believe Achieve Ltd | Other services | 2016 |
| Makiadi Operations Ltd | Construction | 2025 |
| Malevo (Holdings) Limited | Administrative & support | 2013 |
| Malevo Limited | Administrative & support | 2010 |
| Mana Consultancy Ltd | Information & communication | 2023 |
| Mandkhudson Limited | Professional & scientific | 2022 |
| Mandy Consulting Limited | Information & communication | 2010 |
| Mandy Property Holdings Ltd | Real estate | 2024 |
| Mandy's Ironing Service Ltd | Administrative & support | 2023 |
| Mangrove Diner Ltd | Hospitality | 2021 |
| Manmade Construction Ltd | Construction | 2025 |
| Mann Lift Services Ltd | Manufacturing | 2026 |
| Manor Gate Solutions Limited | Transport & logistics | 2015 |
| Manor Green Developments Limited | Construction | 2023 |
| Mantissa Limited | Information & communication | 1981 |
| Maple Carpentry (UK) Limited | Construction | 2011 |
| Maple Inspirations Limited | Administrative & support | 2018 |
| Maplestreet Limited | Real estate | 2016 |
| Maraami Properties Ltd | Real estate | 2022 |
| Marble Coatings Limited | Construction | 2024 |
| Marche Property Group Limited | Real estate | 2021 |
| Marcus Inc Limited | Professional & scientific | 2020 |
| Margaretta Rose Limited | Administrative & support | 2015 |
| Marid Ltd | Construction | 2023 |
| Marie Pascall Limited | Administrative & support | 2010 |
| Maritz Engineering Limited | Construction | 2016 |
| Marius Logistics and Catering Limited | Transport & logistics | 2025 |
| Marjinal Recruitment Ltd | Administrative & support | 2023 |
| Mark Antony and Sons Ltd | Construction | 2022 |
| Mark Luck (Holdings) Limited | Financial services | 2006 |
| Mark Luck Limited | Construction | 1977 |
| Mark Murphy Design Limited | Professional & scientific | 2017 |
| Mark Reilly Limited | Professional & scientific | 2024 |
| Mark Roblin Limited | Administrative & support | 2014 |
| Mark Williams Real Estate Limited | Administrative & support | 2017 |
| MARK1@HOME Solutions Limited | Real estate | 2024 |
| MARK1@SITE Solutions Limited | Construction | 2016 |
| Marlborough Wholesale Ltd | Administrative & support | 2009 |
| Marljac Ltd | Real estate | 2021 |
| Marquis Fire and Security Services Limited | Construction | 2011 |
| Marrabon Close Management Company Limited | Households | 1994 |
| Marsell Limited | Wholesale & retail | 2020 |
| Marsh Choir Ltd | Arts & recreation | 2025 |
| Marshall Investment Group Limited | Financial services | 2015 |
| Martech Electrical Services Limited | Construction | 2019 |
| Martha Russell Limited | Administrative & support | 2019 |
| Martin Bishop Academy Ltd | Arts & recreation | 2008 |
| Martin C Dean Limited | Administrative & support | 2015 |
| Martin Moser Limited | Administrative & support | 2009 |
| Martin Tattoo Limited | Arts & recreation | 2017 |
| MAS Ventilation Services Limited | Construction | 2015 |
| Maseka Limited | Health & social care | 2018 |
| Masons Scaffolding Ltd | Construction | 1993 |
| Master Magic Ltd | Construction | 2025 |
| Mastic Pro Ltd | Construction | 2023 |
| Mater DEI (Sussex) Limited | Extraterritorial | 2013 |
| Materworks Ltd | Arts & recreation | 2006 |
| Matrix-Vision Learning Ltd | Education | 2025 |
| Matt French Ltd | Administrative & support | 2024 |
| Matt Gaster Ltd | Professional & scientific | 2020 |
| Matt Gentle Consulting Limited | Information & communication | 2019 |
| Matthew Brown Building & Maintenance Limited | Administrative & support | 2002 |
| Matthew Ford Limited | Wholesale & retail | 2013 |
| Matthew Senior Music Limited | Administrative & support | 2018 |
| Matthew Terence Consulting Limited | Financial services | 2026 |
| Maui Aesthetics Limited | Other services | 2023 |
| MAX and Co 786 Ltd | Health & social care | 2024 |
| MAX Milner Electrical Limited | Construction | 2013 |
| Maximas Holdings Ltd | Financial services | 2024 |
| Maximas Sidcup Ltd | Real estate | 2024 |
| MAY Fitness Ltd | Administrative & support | 2021 |
| Mayday Rail Services Limited | Construction | 2024 |
| Mayfield Business Consultant Ltd | Information & communication | 2022 |
| Maylands Construction Ltd | Other services | 2014 |
| Maylands Homes Ltd | Real estate | 2019 |
| Maylands Investment Ltd | Real estate | 2013 |
| Maynard Launches Limited | Other services | 1975 |
| Mayrose Rental Limited | Real estate | 2024 |
| Mayvin-Links Limited | Hospitality | 2019 |
| Mazbox Limited | Administrative & support | 2011 |
| MB Construction Services Limited | Construction | 2024 |
| MB Installations Ltd | Construction | 2025 |
| MB Rail & Construction Ltd | Professional & scientific | 2026 |
| MB Rail Access Ltd | Construction | 2022 |
| Mbc-Cleaning Ltd | Administrative & support | 2006 |
| Mbc-Security Limited | Administrative & support | 2007 |
| Mburton Electrical Services Limited | Construction | 2026 |
| MC Contractors (South East) Ltd | Other services | 2022 |
| Mcbrides Assurance Services Limited | Professional & scientific | 2019 |
| Mccarthy's Plumbing Solutions Ltd | Construction | 2025 |
| Mcconville Construction Group Ltd | Construction | 2025 |
| Mccormack Property Holdings Limited | Real estate | 2020 |
| MCD Environmental Ltd | Professional & scientific | 2021 |
| MCG Rail Services Limited | Construction | 2019 |
| MCI Global Ltd | Administrative & support | 2017 |
| Mckinlay Williams Estates Limited | Real estate | 1949 |