| Mark Porter Associates Ltd | Administrative & support | 2011 |
| Mark Smith Architects Limited | Professional & scientific | 2019 |
| Market Chartist Limited | Professional & scientific | 2012 |
| Market Street Clinic (Essex) Limited | Health & social care | 2022 |
| Marketing Consultancy House Limited(The) | Professional & scientific | 1984 |
| Markp Limited | Professional & scientific | 2010 |
| Marriage Property Investment Ltd | Real estate | 2023 |
| Marriott Estates Limited | Construction | 2017 |
| Mars Corporation Limited | Information & communication | 1996 |
| Marsta Residential Ltd | Real estate | 2019 |
| Mascall Homes Limited | Administrative & support | 2005 |
| Masch Chatsworth Ltd | Households | 2019 |
| Masch Developments Ltd | Administrative & support | 2014 |
| Masibalavu Limited | Transport & logistics | 2018 |
| Maslen Homes Limited | Construction | 2019 |
| Mason Lain Properties Limited | Real estate | 1972 |
| Massira Capital Ltd | Financial services | 2023 |
| Massira Estates Ltd | Real estate | 2024 |
| Matara Property Limited | Real estate | 2025 |
| Matchless Marine Limited | Administrative & support | 2005 |
| Mathieson Plural Limited | Administrative & support | 2024 |
| Mathurst Developments Ltd | Construction | 2019 |
| Mathurst Investments Ltd | Real estate | 2019 |
| Mathurst Limited | Construction | 2018 |
| Matlock Coffee Roasters Limited | Manufacturing | 2025 |
| Maurice Close Management Limited | Households | 2005 |
| Mawse Ltd | Administrative & support | 2024 |
| MAX Law Films Limited | Information & communication | 2021 |
| Maxfield & Sons,Limited | Professional & scientific | 1893 |
| Maxwell Drummond Group Holdings Limited | Professional & scientific | 2017 |
| Mayfair Private Office Ltd | Administrative & support | 2019 |
| Mayflower Living Space (MW) Ltd | Administrative & support | 2022 |
| Mayflower Living Space Limited | Construction | 2021 |
| Mbali Ltd | Information & communication | 2018 |
| Mcalister Holdings Limited | Hospitality | 1997 |
| Mcdonald Ingram Limited | Professional & scientific | 2001 |
| Mcdonald Management Artists Limited | Administrative & support | 2024 |
| MCI Consultants Ltd | Information & communication | 2019 |
| MDN Ventures Ltd | Households | 2024 |
| ME and MRS Jones (Saffron Walden) Limited | Wholesale & retail | 2020 |
| ME Associates Ltd | Professional & scientific | 2024 |
| Meadow Rise (South Creake) Management Company Limited | Real estate | 2009 |
| Mechanical Swifts Limited | Information & communication | 2025 |
| Medical Aesthetic Group Ltd | Professional & scientific | 2013 |
| Medical Aesthetic Supplies Limited | Wholesale & retail | 2001 |
| Medistat Software (UK) Ltd. | Information & communication | 1994 |
| Mele Properties Limited | Real estate | 2024 |
| Meliora (UK) Advisory Limited | Professional & scientific | 2025 |
| Mello Publishing Limited | Professional & scientific | 2019 |
| Melodic Productions Limited | Information & communication | 2023 |
| Menorca Yachting Limited | Extraterritorial | 2012 |
| Mentmore Gransden Limited | Construction | 2011 |
| MEP Commercial Services Ltd | Professional & scientific | 2018 |
| Merc Buildings Limited | Real estate | 2013 |
| Mercer Tree Services Ltd | Administrative & support | 2013 |
| Merlins Pet Emporium Limited | Wholesale & retail | 2026 |
| Merrcys Limited | Professional & scientific | 2012 |
| Merrymaking Media Limited | Information & communication | 2016 |
| Mesh and Blade Ltd | Manufacturing | 2024 |
| Mestag Therapeutics Ltd | Professional & scientific | 2020 |
| MET Innovation Limited | Information & communication | 2021 |
| Metalyze Ltd | Professional & scientific | 2025 |
| Metaplan Limited | Construction | 2018 |
| Metmotec Limited | Manufacturing | 2006 |
| Metro Advisory Limited | Administrative & support | 2022 |
| Metropolis Removals Ltd | Transport & logistics | 2021 |
| MGF (Enfield) Ltd | Administrative & support | 2024 |
| Mgid Limited | Professional & scientific | 2005 |
| MGL Carpentry and Construction Limited | Construction | 2017 |
| MI Aviation Holdings Limited | Financial services | 2020 |
| Miconnect Limited | Education | 2017 |
| Midway Atlantic Limited | Arts & recreation | 2015 |
| Mighton Products Limited | Manufacturing | 1996 |
| Milchess Ltd | Professional & scientific | 2020 |
| Mill Scott Ltd | Professional & scientific | 2010 |
| Millfield Electronics Limited | Administrative & support | 1998 |
| Mind Nudger AI Ltd | Professional & scientific | 2022 |
| Mindset Rx'D Ltd | Arts & recreation | 2019 |
| Mindsets (UK) Limited | Manufacturing | 2001 |
| Minet Private Hire Limited | Transport & logistics | 2007 |
| Mini Loaves Limited | Extraterritorial | 2025 |
| Mini Miss Bread Ltd | Manufacturing | 2018 |
| Minimori UK Limited | Wholesale & retail | 2025 |
| Minni Parker Designs Limited | Administrative & support | 2024 |
| Mirror Farm Limited | Agriculture & fishing | 2018 |
| Misa Leisure Ltd | Financial services | 2024 |
| Miss E M Scott Ltd | Health & social care | 2017 |
| Mist Hill Pictures Ltd | Information & communication | 2023 |
| Misway Training Ltd | Professional & scientific | 2024 |
| Mitre Property Development Limited | Construction | 2010 |
| Mivy Limited | Administrative & support | 2023 |
| MJ Family Group Ltd | Professional & scientific | 2019 |
| MKO Aviation Ltd | Manufacturing | 2022 |
| MN Solutions (Cambridge) Limited | Professional & scientific | 2011 |
| Mnai Ltd | Professional & scientific | 2025 |
| Mobile Cleaning Group Limited | Administrative & support | 2026 |
| Mobile Pools UK Ltd | Arts & recreation | 2022 |
| Mocha Cafe Diner Limited | Hospitality | 2007 |
| Moda Kitchen Studio Ltd | Wholesale & retail | 2026 |
| Model Analytics Limited | Professional & scientific | 2014 |