| Madison Cycles Limited | Extraterritorial | 1990 |
| Madison Group Limited | Extraterritorial | 1995 |
| Maefred Consultants Limited | Real estate | 2021 |
| Malamar Homes (Dawley) Limited | Construction | 2006 |
| Malamar Homes (Newport) Limited | Construction | 2005 |
| Malone Roofing (Newbury) Limited | Construction | 1979 |
| Malta Express Limited | Transport & logistics | 2005 |
| Mamatoto Childcare Limited | Professional & scientific | 2005 |
| MAN of Ross House (Management) Limited | Real estate | 1993 |
| MANAGE4U Limited | Professional & scientific | 2004 |
| Manorcott Legal & Business Services LLP | | 2004 |
| Manstar Limited | Information & communication | 1990 |
| Manstones Limited | Real estate | 2006 |
| Mantua Model UK Ltd | Wholesale & retail | 1996 |
| Marazzi (UK) Limited | Construction | 2000 |
| Marfleet Associates Limited | Professional & scientific | 1997 |
| Marginsource Limited | Professional & scientific | 1987 |
| Marie Kaing Consulting Ltd | Professional & scientific | 2019 |
| Mariner Systems (UK) Limited | Information & communication | 1997 |
| Maris Seabed Rigs Limited | Professional & scientific | 2004 |
| Mariya RAA Saddi Ltd | Wholesale & retail | 2026 |
| Marketing House Publishers Limited | Extraterritorial | 1964 |
| Marketing Menu Ltd | Professional & scientific | 2002 |
| Marketing Training Limited | Extraterritorial | 1964 |
| Markus FOX Consulting Limited | Professional & scientific | 1999 |
| Marlin-Europe Limited | | 1997 |
| Marquis Holdings Limited | Administrative & support | 1998 |
| Mars Healthcare Trustees Limited | Professional & scientific | 1996 |
| Mars Nominees Limited | Other services | 2004 |
| Mars Pension Trustees Limited | Professional & scientific | 1948 |
| Martin Welch Ltd | Professional & scientific | 2007 |
| Master Brands Europe Limited | Professional & scientific | 1967 |
| Masters Project Management Consultancy Ltd | Administrative & support | 2025 |
| Matrix Design and Sales Services Limited | Professional & scientific | 2007 |
| Matthew Residential Care Limited | Health & social care | 2003 |
| Matthews Computer Systems Limited | Extraterritorial | 1984 |
| Maximo Computer Services Ltd | Information & communication | 2000 |
| Maya Foundation | Real estate | 2025 |
| Maya UK Group Limited | Financial services | 2024 |
| Mayacare Living Limited | Health & social care | 2020 |
| Mayglade Limited | Manufacturing | 1982 |
| Maylings Jewellers | Wholesale & retail | 1960 |
| MBS Contracting Ltd | Construction | 2018 |
| Mccoy Pharma UK Limited | Extraterritorial | 2019 |
| Mckay Financial Consulting Limited | Professional & scientific | 2002 |
| Mckechnie Leisure and Trading Co. Limited | Real estate | 1980 |
| Mclane Specialist Installation Services Limited | Construction | 1999 |
| MDK Inc Consultants Limited | Information & communication | 2006 |
| Measurement Systems Limited | Manufacturing | 1981 |
| Measurement Systems Trustee Company Limited | Professional & scientific | 1992 |
| Medevale Pharmaservices Limited | Manufacturing | 1918 |
| Medina Transport Limited | Transport & logistics | 2003 |
| Mediplex Limited | Real estate | 1998 |
| Meeya Ltd | Manufacturing | 2001 |
| Melton House Management Limited | Households | 2003 |
| Melvyn Allen Design Limited | Professional & scientific | 2003 |
| Merronsound Limited | Information & communication | 1979 |
| Metawell Limited | Administrative & support | 2016 |
| Metro Products (Accessories & Leisure) Limited | Extraterritorial | 1979 |
| MFM Brands Limited | Wholesale & retail | 2025 |
| MFM Security Services Limited | Administrative & support | 2025 |
| Miah's (Pangbourne) Limited | Real estate | 2002 |
| Miajac Limited | Other services | 2006 |
| Michael Selcott Designs Limited | Extraterritorial | 1977 |
| Micronet Network Solutions Limited | Information & communication | 1999 |
| Midland Angling Products Limited | Extraterritorial | 1984 |
| Midland National Finance Corporation Limited | | 1933 |
| Mighty Digital Online Ltd | Wholesale & retail | 2025 |
| Milltile Limited | Construction | 1990 |
| Millwood Management Company Limited | Households | 1989 |
| Minerva Technologies UK Ltd | Information & communication | 2005 |
| Minster Court (Camberley) Limited | Households | 1964 |
| Mint Contracts Limited | Information & communication | 1994 |
| Mistral Bikes Motorsport | | 2000 |
| Mistral Computing Limited | Information & communication | 1993 |
| Mistry Marketing Services Limited | Administrative & support | 2003 |
| MMA Residentials Ltd | Real estate | 2023 |
| MNA & Sons Limited | Water & waste | 2007 |
| Mobiles and Gadgets Ltd | Wholesale & retail | 2018 |
| Modern Display Organization Limited | Wholesale & retail | 1961 |
| Monaco & British Mercantile Limited | Professional & scientific | 1998 |
| Monica Kinder Consulting Limited | Professional & scientific | 2007 |
| Montmartre Patisserie Limited | Hospitality | 2002 |
| Monze Properties Limited | Real estate | 1999 |
| Moon River Properties Limited | Wholesale & retail | 1972 |
| Morphic Management Limited | Administrative & support | 2004 |
| Moyana Limited | Health & social care | 2018 |
| Msac Consultants Ltd | Information & communication | 2010 |
| Munster Avenue Residents Limited | Real estate | 2006 |
| Music MAN Reading Limited | Wholesale & retail | 2003 |
| Music Tech Training Centre Ltd | Manufacturing | 1999 |
| MV Techsolutions Limited | Information & communication | 2019 |
| Myoken Holdings Ltd | Real estate | 2022 |
| N a Design Ltd | Information & communication | 1999 |
| N T S Associates Limited | Professional & scientific | 2006 |
| N.J. Freight Limited | Transport & logistics | 2005 |
| N2N IT Solutions Limited | Information & communication | 2015 |
| Naheed Charitable Foundation Trust | Professional & scientific | 2006 |
| Napier-Watson Consulting Limited | Professional & scientific | 2002 |
| Nash & Co (Caversham) Limited | Wholesale & retail | 2004 |