| Maria Lancaster Ltd | Health & social care | 2023 |
| Maria Residential Home (EMI) Limited | Health & social care | 2005 |
| Marian Francis Consultancy Ltd | Professional & scientific | 2022 |
| Marie Ange Ltd | Professional & scientific | 2025 |
| Marinette Limited | Health & social care | 2022 |
| Marisol Limited | Health & social care | 2017 |
| Marisol Property Limited | Hospitality | 2025 |
| Mark Blooms Ltd | Administrative & support | 2025 |
| Mark Iliffe Consulting Limited | Administrative & support | 2012 |
| Mark Nichols Plumbing and Heating Ltd | Construction | 2022 |
| Mark Scaffolding Limited | Construction | 2017 |
| Mark Vitow Limited | Wholesale & retail | 1989 |
| Mark Walker Properties Ltd | Real estate | 2020 |
| Marker AI Ltd | Information & communication | 2019 |
| Market Initiatives International Limited | Wholesale & retail | 2001 |
| Market Square Management Limited | Households | 2016 |
| Marketsparksol Ltd | Wholesale & retail | 2026 |
| Marley Gifting Limited | Wholesale & retail | 2025 |
| Marlin Services Limited | Construction | 1981 |
| Marlow Beauty Ltd | Other services | 2019 |
| Marlow Financial Planning Limited | Professional & scientific | 2019 |
| Marlow Grill Ltd | Hospitality | 2025 |
| Marlow Health and Wellness House Limited | Wholesale & retail | 2024 |
| Marlow Healthcare Ltd | Health & social care | 2022 |
| Marlow Homes Limited | Real estate | 1996 |
| Marlow Mead Limited | Extraterritorial | 2019 |
| Marmaris Kebab & Pizza EA Ltd | Hospitality | 2021 |
| Marmat Properties Limited | Administrative & support | 2008 |
| Marrero & Co. Limited | Professional & scientific | 2011 |
| Mars Build Ltd | Construction | 2023 |
| Marsh Business Services Ltd | Administrative & support | 2019 |
| Martar Ltd | Wholesale & retail | 2025 |
| Marteys Homes Ltd | Real estate | 2025 |
| Martial Mind Power Academy Camden Ltd | Health & social care | 2025 |
| Martin (UK) Roofing Systems Limited | Construction | 2009 |
| Martin Green Limited | Wholesale & retail | 2021 |
| Martin-Fagg Associates Limited | Professional & scientific | 2012 |
| Martyn Saunders Builders Limited | Construction | 2013 |
| Martyn’S Miscellaneous Merchandise Ltd | Wholesale & retail | 2023 |
| Marvel Business Limited | Information & communication | 2012 |
| Marvi Developments Ltd | Real estate | 2026 |
| Marwa Services Ltd | Hospitality | 2018 |
| Marx Transport Vehicles Ltd | Transport & logistics | 2003 |
| Mary Cars Limited | Transport & logistics | 2020 |
| Mary&Alex Limited | Transport & logistics | 2015 |
| Maryo&Erica Limited | Construction | 2015 |
| MAS & Sons Ltd | Health & social care | 2019 |
| MAS Scientific LAB Trading Limited | Wholesale & retail | 2025 |
| Masa Organic Supplies Ltd | Wholesale & retail | 2023 |
| Masfa Fragrance Limited | Wholesale & retail | 2025 |
| Masnavi Accountants Ltd | Professional & scientific | 2019 |
| Masnavi Group Ltd | Real estate | 2024 |
| Masonic Regalia Outlet Ltd | Wholesale & retail | 2023 |
| Masonry Management Services Ltd | Construction | 2013 |
| Masonry Milan Ltd | Other services | 2024 |
| Mass Manchester Ltd | Real estate | 2016 |
| Master of Glass Limited | Construction | 2020 |
| Mastercod (Wycombe) Ltd | Hospitality | 2004 |
| Mastering Leadership Skills Limited | Professional & scientific | 2016 |
| Masters of Aesthetics Academy Ltd | Other services | 2018 |
| Mastre Investments Limited | Real estate | 2019 |
| Mat-Solutions Limited | Administrative & support | 2011 |
| Materials Recovery Nominees Limited | Water & waste | 2004 |
| Mathews,Comfort & Co.Limited | Financial services | 1932 |
| Mathseducators Ltd | Education | 2021 |
| Matrech Limited | Real estate | 2014 |
| Matt Fowler Photography Limited | Professional & scientific | 2020 |
| Matt Smith Music Limited | Administrative & support | 2015 |
| Matt Sully Building and Roofing Contractor Ltd | Construction | 2020 |
| Matt Willcock Consulting Ltd | Professional & scientific | 2015 |
| Matt&Brand Solutions Limited | Construction | 2025 |
| Maulden Meadows Management Company Ltd | Households | 2026 |
| Mauler MMA Ltd | Manufacturing | 2018 |
| Mava Electrical Services Ltd | Construction | 2019 |
| Maven Property Ltd | Real estate | 2024 |
| Mavens Developments Ltd | Construction | 2021 |
| Mavens Tech Ltd | Information & communication | 2013 |
| Mawgan Stays (Cornwall) Ltd | Hospitality | 2021 |
| MAX Power Electrics Limited | Manufacturing | 2024 |
| MAX Powergas Limited | Construction | 2014 |
| Maxcare Health Limited | Health & social care | 2013 |
| Maxi Rides Ltd | Wholesale & retail | 2022 |
| Maximo Development Limited | Real estate | 2006 |
| Maximon Solutions Limited | Information & communication | 2000 |
| Maximy Limited | Education | 2015 |
| Maxma Ltd | Health & social care | 2025 |
| Maxsonic Ltd | Wholesale & retail | 2020 |
| MAY Wellbeing Ltd | Health & social care | 2013 |
| Mayamer Ltd | Information & communication | 2025 |
| Mayfair Kitchen and Bedrooms Limited | Construction | 2022 |
| Mayfield Electrical Services Limited | Construction | 2003 |
| Mayfield Manor Ltd | Health & social care | 2026 |
| Mayflair Media Limited | Information & communication | 2026 |
| Mayflies Properties Limited | Real estate | 2021 |
| Mayhew Building Services Ltd | Manufacturing | 2019 |
| Mayhew Crescent Homes Ltd | Real estate | 2022 |
| Mayrakee Ltd | Public administration | 2022 |
| MB & Partners Limited | Information & communication | 2000 |
| MB Digital Services Ltd | Professional & scientific | 2025 |
| MBA CM Limited | Construction | 2018 |