| Maple House (Shenfield) Residents Association Limited | Real estate | 1987 |
| Mapleton Growers Ltd | Agriculture & fishing | 2000 |
| Maralesse Limited | Professional & scientific | 2010 |
| March Hare Data Limited | Administrative & support | 2017 |
| March Osteopathy Limited | Health & social care | 2014 |
| Margaret Adams Ltd | Health & social care | 2025 |
| Marillac Neurological Care Centre | Health & social care | 2019 |
| Marine Engineering Technology Ltd | Professional & scientific | 2019 |
| Mark Costello Consultancy Ltd | Professional & scientific | 2020 |
| Mark Costello Costume Ltd | Manufacturing | 2020 |
| Mark Peter Ltd | Real estate | 2018 |
| Mark Thompson Productions Ltd | Other services | 2013 |
| Mark Thompson Sportswear Ltd | Wholesale & retail | 2025 |
| Market Fresh Limited | Wholesale & retail | 2005 |
| Market Store at B Limited | Wholesale & retail | 2019 |
| Marlborough Court (Queens Road) Limited | Real estate | 1995 |
| Marlborough Property Maintenance Limited | Construction | 2023 |
| Marlow Montessori School Ltd | Health & social care | 2023 |
| Marlowe Road Management Company Limited | Households | 2021 |
| Marmion Mews Management Ltd | Households | 2006 |
| Marood Architecture Ltd | Professional & scientific | 2021 |
| Marood Properties Ltd | Real estate | 2018 |
| Marrco 25 Limited | Construction | 2011 |
| Mars Bidco Limited | Financial services | 2019 |
| Mars Debtco Limited | Financial services | 2019 |
| Mars Finco Limited | Financial services | 2019 |
| Mars Holdings Group Limited | Financial services | 2026 |
| Mars Midco Limited | Financial services | 2019 |
| Marshall Group Investments Ltd | Real estate | 2026 |
| Marshall-Wells Digital Innovations & Technology Ltd | Information & communication | 2025 |
| Martial Arts 4 ALL C.I.C. | Education | 2023 |
| Martin Olsson Ltd | Wholesale & retail | 2009 |
| Martlew Knight Limited | Professional & scientific | 2025 |
| Martongate Limited | Administrative & support | 1992 |
| Martyn LEE Construction Ltd | Construction | 2021 |
| Marvin Home Ltd | Real estate | 2021 |
| Marybeth Limited | Real estate | 2020 |
| Maryirv Ltd | Health & social care | 2015 |
| Mascalls Farms LLP | | 2025 |
| Mascalls Finance Limited | Other services | 2022 |
| Maskill Mundy Limited | Real estate | 2021 |
| Massha Limited | Health & social care | 2017 |
| Massha Property Ltd | Real estate | 2023 |
| Master Class Education Ltd | Administrative & support | 2007 |
| Master John's Ltd | Professional & scientific | 2005 |
| Masterplus Limited | Real estate | 1993 |
| Masterwright Properties Limited | Real estate | 2026 |
| Match ME Recruitment Ltd | Administrative & support | 2015 |
| Matchroom Boxing Global LLP | | 2018 |
| Matchroom Boxing Limited | Information & communication | 2016 |
| Matchroom Boxing USA Limited | Information & communication | 2018 |
| Matchroom Charitable Foundation | Professional & scientific | 2016 |
| Matchroom Holdings Limited | Financial services | 2021 |
| Matchroom Media Ltd | Information & communication | 2021 |
| Matchroom Multi Sport Limited | Information & communication | 2020 |
| Matchroom Sport Limited | Information & communication | 1982 |
| Matharu Estates LLP | | 2020 |
| Matrix IT Solution Limited | Wholesale & retail | 2014 |
| Mattabolism Personal Training Limited | Arts & recreation | 2012 |
| Matthew Galea Health & Lifestyle Ltd | Other services | 2024 |
| Matthew Luke Ltd | Information & communication | 2020 |
| Matthias Associates Limited | Professional & scientific | 2006 |
| Maui Bidco Limited | Financial services | 2019 |
| Mavdale Limited | Real estate | 2025 |
| MAX Building & Renovation Ltd | Construction | 2020 |
| MAX Whitlock Gymnastics Ltd | Education | 2017 |
| Max-Tm Ltd | Other services | 2006 |
| MAXCAR1 Limited | Wholesale & retail | 2016 |
| Maxi Delight Limited | Hospitality | 2023 |
| Maxira Consulting Ltd | Information & communication | 2014 |
| Maxivent (Electrical & Ventilation) Ltd. | Construction | 2015 |
| Maxtracker Ltd | Transport & logistics | 2009 |
| Maxway Investments Limited | Real estate | 1995 |
| Maxxpal Ltd | Information & communication | 2020 |
| Maybells Management Services Limited | Real estate | 1966 |
| Mayes Glaze Ltd | Construction | 2018 |
| Mayfair Grace Ltd | Other services | 2026 |
| Mayfair Labs Ltd | Information & communication | 2026 |
| Mayfest 30 Worldwide Limited | Professional & scientific | 2024 |
| Mayfield Building Services Limited | Construction | 2002 |
| Mayfield Service (London) Limited | Construction | 2024 |
| Mayhew Mechanical Ltd | Construction | 2026 |
| Mazacroft Limited | Professional & scientific | 2012 |
| Mazo Consultancy Ltd | Professional & scientific | 2023 |
| Mazo Limited | Administrative & support | 2017 |
| Mazo Properties Limited | Administrative & support | 2018 |
| Mazv Services Ltd | Construction | 2019 |
| MB Architectural Drawings Limited | Professional & scientific | 2014 |
| MB Business Consulting Ltd | Professional & scientific | 2023 |
| MB Eastcote Limited | Other services | 2020 |
| MB Marketing Group Ltd | Professional & scientific | 2021 |
| MB Metalworks Limited | Construction | 2016 |
| MB Property Group Ltd | Real estate | 2022 |
| MB Revive Property Services Ltd | Construction | 2026 |
| MBK CS Ltd | Administrative & support | 2019 |
| MBL Building Services Ltd | Construction | 2021 |
| Mbsces Limited | Real estate | 2022 |
| MC Recruit Limited | Administrative & support | 2018 |
| MC RTM Company Limited | Households | 2021 |
| Mcardle Interiors Limited | Construction | 2013 |