| Magnetar Ltd | Information & communication | 2011 |
| Maison D'Or Events Ltd | Professional & scientific | 2025 |
| Maison DE LUX Ltd | Administrative & support | 2025 |
| Maison Property Services AP Ltd | Construction | 2021 |
| Maison Xcape Ltd | Hospitality | 2021 |
| Majakas Ltd | Wholesale & retail | 2024 |
| Maklane Limited | Transport & logistics | 2026 |
| Maltings (Sawbridgeworth) Management Company Limited(The) | Households | 1986 |
| Mamma K'S Kitchen Ltd. | Hospitality | 2020 |
| Management and Business Restructuring Ltd | Professional & scientific | 2010 |
| Manor Cars Potters Bar Ltd | Transport & logistics | 2016 |
| Manor Estates Lettings Limited | Real estate | 2012 |
| Manor Properties (Chigwell) Limited | Real estate | 1994 |
| Maqdad Holdings Ltd | Financial services | 2025 |
| Marart Ltd | Construction | 2015 |
| Maratsis Consulting Ltd | Information & communication | 2017 |
| Marble & Joinery Workshop Limited | Construction | 2011 |
| Marco Capaldo Accessori Limited | Administrative & support | 2015 |
| Marco Capaldo Accessories Limited | Administrative & support | 2015 |
| Marco Capaldo Casa Limited | Administrative & support | 2015 |
| Marco Capaldo Couture Limited | Administrative & support | 2015 |
| Marco Capaldo Home Limited | Administrative & support | 2015 |
| Marco Capaldo Homme Limited | Administrative & support | 2015 |
| Marco Capaldo International Limited | Administrative & support | 2015 |
| Marco Capaldo LE Parfum Limited | Administrative & support | 2015 |
| Marco Capaldo Limited | Administrative & support | 2015 |
| Marco Capaldo Profumi Limited | Administrative & support | 2015 |
| Marco Capaldo Spa Limited | Administrative & support | 2015 |
| Marco Capaldo Uomo Limited | Administrative & support | 2015 |
| Marco the Malta BUS Limited | Information & communication | 2013 |
| Margo Properties Ltd | Real estate | 2021 |
| Maria Zecca Financial Planning Limited | Professional & scientific | 2020 |
| Marilyns Events Ltd | Administrative & support | 2011 |
| Marine and Overseas Services (1939) Limited | Professional & scientific | 1939 |
| Maritina Limited | Other services | 2007 |
| Mariva Consulting Ltd | Administrative & support | 2015 |
| Marivem Ltd | Administrative & support | 2016 |
| Market Place Management (Hertford) Limited | Real estate | 2007 |
| Market Place Property Investment Limited | Real estate | 2023 |
| Marlex Lodge Freehold Limited | Other services | 2020 |
| Marlex Lodge Limited | Households | 1979 |
| Marlingrose Limited | Administrative & support | 2016 |
| Marquee Commercial Services Ltd | Construction | 2026 |
| Marquee Contracting Limited | Construction | 2014 |
| Marquee Operations Limited | Construction | 2017 |
| Marquee PAY Ltd | Administrative & support | 2026 |
| Marriott Enterprises Limited | Professional & scientific | 2005 |
| Martin Mills Consultants Limited | Professional & scientific | 2018 |
| Martin Pitt Consulting Limited | Professional & scientific | 2004 |
| Martin Shepherd Solicitors LLP | | 2010 |
| Maryam Express Limited | Hospitality | 2018 |
| Marylebone Clinic Ltd | Administrative & support | 2023 |
| Marylebone Property Developments Limited | Construction | 2019 |
| Mass Consulting Group Ltd | Other services | 2023 |
| Mass Hallucination Limited | Arts & recreation | 2013 |
| Massive Attack Live Ltd | Arts & recreation | 2014 |
| Massive Live 2013 Limited | Arts & recreation | 2013 |
| Master Management Limited | Professional & scientific | 2002 |
| Matgas Works Ltd | Construction | 2015 |
| Maths Alpha Limited | Education | 2024 |
| Mathstrainer Ltd | Information & communication | 2025 |
| Matrix Electrical Engineering Limited | Construction | 2009 |
| Mattmist Property Maintenance Ltd | Administrative & support | 2016 |
| Maverick Camper Collective Ltd | Wholesale & retail | 2026 |
| Maverick Trading Limited | Administrative & support | 2014 |
| Maxi Store Ltd | Wholesale & retail | 2018 |
| Maximus Corporate Services Ltd | Administrative & support | 2016 |
| Maxkold Holdings Ltd | Financial services | 2023 |
| Mayfield Property Developments Limited | Construction | 2016 |
| MAZ Beauty Ltd | Wholesale & retail | 2021 |
| Mazamo Limited | Administrative & support | 2015 |
| Mcadom Limited | Health & social care | 2024 |
| Mccracken Build Co. Ltd | Construction | 2025 |
| Mccracken Roofing Services Ltd | Construction | 2020 |
| Mcgarry Construction Limited | Construction | 2008 |
| Mcgarry Rail Ltd | Administrative & support | 2018 |
| Mcintyre Electrical Services Limited | Construction | 2020 |
| MCK Advisory Ltd | Professional & scientific | 2024 |
| Mckeever Investments Ltd | Real estate | 2022 |
| MCM Scaffolding Solutions Ltd | Construction | 2021 |
| Mcmelange Ltd | Wholesale & retail | 2024 |
| MCN Accountants Ltd | Professional & scientific | 2020 |
| MCN Stainless Holdings Limited | Financial services | 2025 |
| Mcnamara/Troy Ltd | Administrative & support | 2016 |
| MCR Beat Limited | Information & communication | 2020 |
| MCR United Ltd | Wholesale & retail | 2017 |
| MD Dental Limited | Health & social care | 2024 |
| MD2 Retail Ltd | Wholesale & retail | 2025 |
| MD3 Retail Ltd | Wholesale & retail | 2025 |
| MDB Stud Limited | Other services | 2021 |
| MDC Motors Limited | Wholesale & retail | 2026 |
| MDP Communications Ltd | Administrative & support | 2025 |
| MDP Communications Services Ltd | Administrative & support | 2007 |
| ME Facilities Management Ltd | Real estate | 2023 |
| ME Shutters & Services Ltd | Wholesale & retail | 2020 |
| MEA Consulting Services Ltd | Professional & scientific | 2018 |
| Meadow Maids Cleaning Service Ltd | Administrative & support | 2024 |
| Meadow Vale (Buntingford) Management Company Limited | Households | 2016 |
| Measured Value Ltd | Extraterritorial | 2023 |
| Mectrical Recruitment Ltd | Administrative & support | 2022 |