| Mars Automotive Ltd | Wholesale & retail | 2021 |
| Marsh Game Limited | Agriculture & fishing | 2016 |
| Marsh House Property Management Limited | Real estate | 2016 |
| Marshland Clean & Care Ltd | Administrative & support | 2025 |
| Marshland Flowerbulbs Limited | Wholesale & retail | 2006 |
| Marshland Telecomm Limited | Information & communication | 1995 |
| Martin Boon Autos Ltd | Wholesale & retail | 2019 |
| Martin Skip Hire Limited | Water & waste | 2023 |
| Martin's Taxis Ltd | Transport & logistics | 2024 |
| Martrans 24 Ltd | Transport & logistics | 2020 |
| Marvel Rooms Ltd | Wholesale & retail | 2024 |
| Mary Peirce Limited | Manufacturing | 2016 |
| Marymatha Group Limited | Health & social care | 2022 |
| Mason Electrical Ltd | Construction | 2026 |
| Massen Properties Limited | Construction | 2017 |
| Massif Media Ltd | Administrative & support | 2025 |
| Masterclass EDU Ltd | Education | 2025 |
| Matrexed Property Holdings Ltd | Real estate | 2025 |
| Matt Cross Lighting Ltd | Arts & recreation | 2024 |
| Matt King Limited | Construction | 2015 |
| Matt Sturgeon Limited | Construction | 2015 |
| Mattchpatch Ltd | Wholesale & retail | 2024 |
| Maximum Results Health & Fitness Limited | Arts & recreation | 2017 |
| Maximum Results Wellness Clinic Limited | Health & social care | 2024 |
| Maxlume Ltd | Wholesale & retail | 2020 |
| Maxogun Ltd | Administrative & support | 2019 |
| Maxse Associates Limited | Professional & scientific | 2021 |
| Maxview Limited | Manufacturing | 1964 |
| MAY76 Ltd | Professional & scientific | 2020 |
| Maya Bistro Ltd | Hospitality | 2023 |
| Maya's Cleaning Bees Ltd | Administrative & support | 2025 |
| Mayfair Properties Norfolk Ltd | Real estate | 2024 |
| Mayflower Renovations Limited | Real estate | 2011 |
| Mayhew Steel Ltd | Manufacturing | 2004 |
| Maze Media Limited | Professional & scientific | 2014 |
| Maze Media Services LLP | | 2009 |
| MB Industrial Roofing Limited | Construction | 2003 |
| MBN Property Developments Limited | Administrative & support | 2020 |
| MC Suited Electrical and Maintenance Ltd | Construction | 2025 |
| Mcanulty Consulting Limited | Administrative & support | 2008 |
| Mcas Associates Ltd | Professional & scientific | 2012 |
| Mccreadie & Schaeffer Limited | Administrative & support | 2014 |
| MCD Properties Ltd. | Construction | 2015 |
| Mcdonnell Gardening and Property Services Ltd | Administrative & support | 2020 |
| Mcdonnell Leisure Limited | Financial services | 2008 |
| MCE Heatcool Ltd | Manufacturing | 2018 |
| Mckeever ST Lawrence Limited | Extraterritorial | 2000 |
| Mckenna Homes Ltd | Real estate | 2025 |
| Mcnulty FBA Ltd | Wholesale & retail | 2025 |
| Mcoadechi Ltd | Health & social care | 2025 |
| MD Garnett Shellfish Limited | Agriculture & fishing | 2015 |
| MDJ Consultancy Services Ltd | Professional & scientific | 2013 |
| MDM Advices Limited | Administrative & support | 2026 |
| Meadow Coffee Roasters Ltd | Manufacturing | 2026 |
| Meadow Gardens Management Limited | Households | 2021 |
| Meadow Market Ltd | Wholesale & retail | 2025 |
| Meadow View Homes (Roydon) Limited | Real estate | 2022 |
| Mechanised Farming Equipment Limited | Agriculture & fishing | 1947 |
| Medapp UK Ltd | Information & communication | 2018 |
| Medenta Laboratories Limited | Manufacturing | 2009 |
| Medikarp Ltd | Health & social care | 2022 |
| Mek-Elek Ltd | Manufacturing | 2020 |
| Mel's Joinery Limited | Manufacturing | 2003 |
| Melisa Property Ltd | Real estate | 2025 |
| Merch (NWI) Ltd | Manufacturing | 2022 |
| Meredy Limited | Health & social care | 2022 |
| Mermaid Fsqr Consultancy Ltd | Wholesale & retail | 2026 |
| Merton Park Ltd | Construction | 2003 |
| Merxin Ltd | Professional & scientific | 2015 |
| Metaclarity Limited | Information & communication | 2000 |
| Metaplast Limited | Wholesale & retail | 2016 |
| Metric Carpets (KL) Limited | Wholesale & retail | 2003 |
| Metric Carpets of Norfolk Ltd | Construction | 2011 |
| MG Building Maintenance Ltd | Construction | 2025 |
| MGK Legal & Business Consulting Ltd | Professional & scientific | 2024 |
| Mgpd Finance Limited | Financial services | 2019 |
| MHM Richmond Limited | Health & social care | 2024 |
| MHP Creative Ltd | Information & communication | 2012 |
| Miamifriedlynn Ltd | Hospitality | 2025 |
| Mian GUL H Limited | Construction | 2025 |
| Michael Cale Consulting Limited | Professional & scientific | 2014 |
| Michael Tighe Plant Hire Limited | Administrative & support | 2001 |
| Michau Studio Ltd | Professional & scientific | 2025 |
| Micklefield Park Limited | Hospitality | 2023 |
| Mickram Limited | Construction | 2005 |
| Microcad Engineering Limited | Administrative & support | 2007 |
| Microfiltrex Limited | Extraterritorial | 1974 |
| Middleton Hall Management Company Limited | Households | 2005 |
| Middleton Towers Railway Station Community Interest Company | Arts & recreation | 2024 |
| Middletons SHG Limited | Hospitality | 2023 |
| Midpoint Contracts Ltd | Construction | 2025 |
| Miguel Cabaco Ltd | Transport & logistics | 2025 |
| Mike Authers Classics Ltd | Wholesale & retail | 2015 |
| Mike Gilbert Building Services Limited | Construction | 2004 |
| Mikey's Auto Services Ltd | Wholesale & retail | 2024 |
| Mila Shop Ltd | Wholesale & retail | 2024 |
| Mill Lane Cottages Management Company Limited | Households | 1978 |
| Miller's Vanguard Limited | Professional & scientific | 1966 |
| Millford House Properties Limited | Real estate | 2018 |
| Millione Limited | Manufacturing | 2011 |