| Marantia Ltd | Wholesale & retail | 2026 |
| Marbrim Property Management Limited | Households | 1985 |
| Marchend Motors Limited | Wholesale & retail | 2006 |
| Marchfield Estates (North) Limited | Construction | 1997 |
| Marchfield Estates (South) Limited | Construction | 1997 |
| Marchfield Estates (West) Limited | Construction | 1997 |
| Marchwood Aggregates Limited | Mining & quarrying | 1993 |
| Marchwood Recycling Limited | Mining & quarrying | 2012 |
| Marcin UK Transport Limited | Transport & logistics | 2016 |
| Marcus John Haynes Ltd | Extraterritorial | 2024 |
| Mardale Industrial Services Limited | Manufacturing | 2001 |
| Mardigrove Limited | Construction | 1974 |
| Mardon Enterprises Limited | Construction | 1982 |
| Mardon Property Services Ltd | Real estate | 2008 |
| Margam Slag Company Limited | Professional & scientific | 1959 |
| Marian Rata Driver Ltd | Transport & logistics | 2015 |
| Mariansolutions Ltd | Construction | 2025 |
| Marie Global Services Ltd | Manufacturing | 2026 |
| Marie Health Ltd | Health & social care | 2020 |
| Marie's Hair & Beauty Salon Ltd | Other services | 2013 |
| Marigold Indian Restaurant UK Limited | Hospitality | 2019 |
| Mario Garage Ltd | Other services | 2019 |
| Mario Insulation Ltd. | Construction | 2012 |
| Mario Tower Ltd | Construction | 2025 |
| Mario's Emporium Ltd | Wholesale & retail | 2025 |
| Mariola's Bites Limited | Hospitality | 2025 |
| Mariolodge Ltd | Construction | 2020 |
| Maritz Events Ltd | Administrative & support | 2025 |
| Mari’S Home Limited | Construction | 2023 |
| Mark 2 Heating and Plumbing Limited | Construction | 2008 |
| Mark Lacey Office Interiors Limited | Construction | 2002 |
| Mark Wildman Ltd | Construction | 2013 |
| Market Cross Property Limited | Other services | 2018 |
| Market Drayton Properties Limited | Real estate | 2012 |
| Market Moke Limited | Professional & scientific | 2026 |
| Market System 2019 Ltd | Wholesale & retail | 2019 |
| Marketing Distribution Services (UK) Limited | Information & communication | 2009 |
| Marketing Klick Ltd | Information & communication | 2024 |
| Marketnest Ltd | Wholesale & retail | 2024 |
| Markfield Technical Solutions Limited | Professional & scientific | 2025 |
| Markham Hospitality & Entertainments Limited | Construction | 2023 |
| Marklix Ltd | Wholesale & retail | 2026 |
| Marnel Court Residents Limited | Households | 1973 |
| Marok Civil Engineering Ltd | Construction | 2020 |
| Marok Constructions Ltd | Construction | 2014 |
| Marok Hire Ltd | Administrative & support | 2018 |
| Marok Properties Ltd | Real estate | 2019 |
| Marok Trading Ltd | Hospitality | 2024 |
| Marqueeyouroccasion Ltd | Administrative & support | 2020 |
| Marsh Delivery Solutions Ltd | Transport & logistics | 2025 |
| Marsh Electrical Services Limited | Construction | 2009 |
| Marshall Capital Holdings Ltd | Financial services | 2026 |
| Marshall Strategic Capital Limited | Financial services | 2026 |
| Marshdale Construction Limited | Construction | 2010 |
| Marshdale Group Ltd | Financial services | 2013 |
| Marshz Ltd | Information & communication | 2021 |
| Marston's Acquisitions Limited | Manufacturing | 1887 |
| Marston's Corporate Holdings Limited | Professional & scientific | 2018 |
| Marston's Estates Limited | Hospitality | 1949 |
| Marston's Operating Limited | Professional & scientific | 2000 |
| Marston's PLC | Hospitality | 1890 |
| Marston's Property Developments Limited | Professional & scientific | 2004 |
| Marston's Pubs Limited | Hospitality | 2005 |
| Marston's Pubs Parent Limited | Professional & scientific | 2005 |
| Marston's Telecoms Limited | Information & communication | 2000 |
| Marston's Trading Limited | Professional & scientific | 1894 |
| Marston's Trustees Limited | Professional & scientific | 2008 |
| Marston, Thompson & Evershed Limited | Extraterritorial | 1896 |
| Martauz Ltd | Transport & logistics | 2012 |
| Marthall Ltd | Transport & logistics | 2022 |
| Marthas Cafe Limited | Hospitality | 2012 |
| Martin & Sidhu Investments Limited | Real estate | 2019 |
| Martin and Monica Care Services Ltd | Administrative & support | 2022 |
| Martin Barlow Welding & Inspection Services Limited | Manufacturing | 2015 |
| Martin Clarke Electrical Installations Ltd | Construction | 2014 |
| Martin Dodd Limited | Professional & scientific | 2009 |
| Martin Evans Construction Limited | Extraterritorial | 1993 |
| Martin Ford Limited | Construction | 2002 |
| Martin&William Consulting Limited | Professional & scientific | 2026 |
| Martina Grace Ltd | Health & social care | 2020 |
| Martina Rose Investments Ltd | Real estate | 2016 |
| Martina Rose Management Services Ltd | Real estate | 2018 |
| Martini Tyre Services Limited | Wholesale & retail | 2005 |
| Martol Holdings Limited | Real estate | 2002 |
| Martol Industries Limited | Administrative & support | 1998 |
| Martwin Electrical Ltd | Construction | 2024 |
| Martworld Care Services Limited | Other services | 2013 |
| Martynslaw Services Ltd | Professional & scientific | 2026 |
| Maru Automotive Ltd | Manufacturing | 2024 |
| Marvellous Finds Limited | Wholesale & retail | 2025 |
| Marvlux Ltd | Wholesale & retail | 2026 |
| Marwah GP Ltd | Health & social care | 2020 |
| Marwall Homes Ltd | Real estate | 2024 |
| Mary Hanah Cameron Limited | Extraterritorial | 2025 |
| Maryandjerryproperties Limited | Real estate | 2020 |
| Marygold Trading Ltd | Construction | 2010 |
| Marylebone Enterprise Holdings (UK) Limited | Health & social care | 2012 |
| MAS Healthcare Limited | Health & social care | 2019 |
| Masair Conditioning & Refrigeration Ltd | Professional & scientific | 2025 |
| Masakhane Zinini Community Ltd | Health & social care | 2020 |