| Marg Property Services Limited | Real estate | 2021 |
| Marini Assets Limited | Professional & scientific | 2025 |
| Mark and Ride Ltd | Transport & logistics | 2024 |
| Market Square Management Company (Staines) Limited | Households | 1979 |
| Marmalite Technologies Ltd | Information & communication | 2022 |
| Marmar 1 Ltd | Administrative & support | 2010 |
| Marmar UK Ltd | Wholesale & retail | 2009 |
| Marok Builders & Construction Ltd | Construction | 2016 |
| Marriott's Events Ltd | Hospitality | 2022 |
| Marriott's Fine Dining Ltd | Hospitality | 2014 |
| Mars and PAX Ltd | Other services | 2021 |
| Mars Construction Limited | Construction | 2021 |
| Marsh Consulting (Ascot) Limited | Administrative & support | 2009 |
| Marshall Arts Office Schematics Ltd Ltd | Administrative & support | 2025 |
| Martenhale Ltd | Construction | 2020 |
| Martha'Sphysiotherapyoxford Limited | Other services | 2024 |
| Masonwood Design Ltd | Professional & scientific | 2020 |
| Mat's Facades Limited | Construction | 2014 |
| Matha Ltd | Information & communication | 2020 |
| Matthew Court (Ashford) Residents Association Limited | Households | 1991 |
| Matthew Jensen Consulting Limited | Information & communication | 2025 |
| Matthews Medical Ltd | Health & social care | 2025 |
| Maxbuild-Uk Limited | Construction | 2025 |
| Maxdwell Limited | Financial services | 2022 |
| Maximum ROI Ltd | Financial services | 2024 |
| Mayfield Park Homes Limited | Construction | 2021 |
| Mayflower Childminding Limited | Other services | 2016 |
| MB Catering Services Ltd | Other services | 2017 |
| Mbb-It Ltd | Information & communication | 2018 |
| MBC Technology Ltd | Information & communication | 2022 |
| MBS Accountants Limited | Professional & scientific | 2023 |
| MC Donald Ltd | Construction | 2019 |
| MCH Entertainment Ltd | Information & communication | 2023 |
| MCI Trading Academy Limited | Education | 2021 |
| MD Flegg Electrical Limited | Administrative & support | 2016 |
| MDC Logistic Ltd | Transport & logistics | 2015 |
| Me&D Ltd | Wholesale & retail | 2016 |
| Meadow Court Freehold (Staines) Limited | Administrative & support | 2011 |
| Meadow View Residents Tamworth Limited | Households | 2025 |
| Mecan Trading Limited | Wholesale & retail | 2026 |
| Mecoserv Limited | Information & communication | 2014 |
| Medhavi Ltd | Real estate | 2023 |
| Medical Emergency Solutions Limited | Transport & logistics | 2019 |
| Mehomia 76 Ltd | Manufacturing | 2017 |
| Mejuicer Ltd | Manufacturing | 2021 |
| Melba One Limited | Professional & scientific | 2011 |
| Melville Marketing Ltd | Administrative & support | 2022 |
| Mental Health in Recruitment Ltd | Other services | 2020 |
| Mercanta Limited | Other services | 1996 |
| Meridian Park (Amenities) Limited | Real estate | 1990 |
| Meridian Property Associates Limited | Real estate | 2015 |
| Mermezan Ltd | Transport & logistics | 2025 |
| Mesmerise Media Limited | Information & communication | 2020 |
| Messums Creative Limited | Arts & recreation | 2024 |
| Messums East Ltd | Wholesale & retail | 2021 |
| Messums Ltd | Wholesale & retail | 2023 |
| Messums Wiltshire Ltd | Wholesale & retail | 2015 |
| Metal Gates Limited | Manufacturing | 2017 |
| Metal Tekh Ltd | Construction | 2025 |
| Methven Builders Limited | Construction | 2004 |
| Metro Prepaid Limited | Other services | 2017 |
| MG Homes London Ltd | Construction | 2025 |
| MG Scenery Ltd | Manufacturing | 2021 |
| MHJ Consultancy Ltd | Professional & scientific | 2021 |
| Michael James Hall GAS Ltd | Construction | 2026 |
| Micoco Cafeteria Ltd | Hospitality | 2025 |
| Micococafe Ltd | Hospitality | 2024 |
| Micrologic Property Holdings Limited | Real estate | 2000 |
| Mid-Tech Engineering Projects Ltd | Construction | 2015 |
| Mid-Tech Services Limited | Construction | 2008 |
| Mida Consultancy Services Ltd | Professional & scientific | 2007 |
| Middlecot Stables Ltd | Agriculture & fishing | 2019 |
| Middlesex Metal Re-Cycling Ltd | Professional & scientific | 2012 |
| Migazoink Ltd | Construction | 2025 |
| Milaahi Ltd | Health & social care | 2024 |
| Mildura Management Ltd | Households | 2015 |
| Miles Consular Services Ltd | Administrative & support | 2020 |
| Milken Europe Limited | Professional & scientific | 2016 |
| Mill Autos Limited | Extraterritorial | 2022 |
| Mille Mills Ltd | Extraterritorial | 2012 |
| Millenium Executive Cars Limited | Administrative & support | 1997 |
| Millennium Executive Cars Limited | Professional & scientific | 2003 |
| Milton Holdco Ltd | Information & communication | 2024 |
| Minar Ltd | Wholesale & retail | 2025 |
| Mini Bears Childcare Ltd | Health & social care | 2024 |
| Mini Marvel Events Ltd | Hospitality | 2025 |
| Minitots Day Care Limited | Administrative & support | 2009 |
| Minster Worldwide Logistics Limited | Transport & logistics | 2008 |
| MINT55 Limited | Professional & scientific | 2021 |
| Minutiae Limited | Information & communication | 2012 |
| Mirafeline Limited | Wholesale & retail | 2026 |
| Miro Evgeniev Ltd | Hospitality | 2018 |
| Mission Care Ltd | Health & social care | 2025 |
| Mistik Fashion Ltd | Wholesale & retail | 2026 |
| Mitakeaway Limited | Information & communication | 2022 |
| Mitchell Plumbing & Heating Services Ltd | Construction | 2019 |
| Mitchell-Holmes Property Investments Ltd | Real estate | 2024 |
| Mitchla Property Management Limited | Construction | 2026 |
| Mitsubishi HC Capital UK PLC | Financial services | 1982 |
| MJ Soft Drinks Ltd | Wholesale & retail | 2021 |