| Mansion House Property Limited | Real estate | 2015 |
| Mansion House Publishing (UK) Ltd | Information & communication | 2011 |
| Mantas Scaffolding Ltd | Construction | 2026 |
| Mantec Innovations Holdings Limited | Financial services | 2022 |
| Mantec Innovations Limited | Construction | 2015 |
| Manticore Games Limited | Wholesale & retail | 2024 |
| Mantix Group Ltd | Information & communication | 2026 |
| Mantrovas Transport Ltd | Transport & logistics | 2017 |
| Manufacturing Efficiency Solutions Ltd | Professional & scientific | 2018 |
| Manvish Ltd | Other services | 2025 |
| MAP Health Holdings Limited | Health & social care | 2023 |
| Mapingo Limited | Information & communication | 2011 |
| Maples Developments Limited | Construction | 2004 |
| Maplestone Homes Limited | Construction | 2016 |
| Maptora Info Struct Ltd | Information & communication | 2026 |
| Mapwhizz Limited | Information & communication | 2025 |
| MAR Casa (Imperial) Limited | Construction | 2025 |
| Marabathuni Limited | Wholesale & retail | 2025 |
| Maravida Voyages Ltd | Transport & logistics | 2025 |
| Marbleman Properties Limited | Real estate | 1984 |
| Marcatapop Ltd | Transport & logistics | 2021 |
| March Consulting (East Anglia) Limited | Professional & scientific | 2004 |
| March Made Ltd | Manufacturing | 2026 |
| Marchdell Limited | Real estate | 1973 |
| Marcktrans Ltd | Transport & logistics | 2024 |
| Marco JR Digital Ventures Ltd. | Information & communication | 2014 |
| MARCOS2017 Ltd | Transport & logistics | 2017 |
| Marcus James Cars Limited | Wholesale & retail | 2016 |
| Margalla Trading Limited | Wholesale & retail | 2025 |
| Margaret's Ecology Limited | Professional & scientific | 2009 |
| Marhaba Group Ltd | Construction | 2022 |
| Maria's Meal House Ltd | Hospitality | 2024 |
| Mariaflores Limited | Wholesale & retail | 2024 |
| Marian Bogdan Ltd | Transport & logistics | 2019 |
| Marimarx Corporation Ltd | Financial services | 2023 |
| Marina (EN) Ltd | Hospitality | 2023 |
| Marina Close (Thetford) Block Management Company Limited | Households | 2022 |
| Marina Education Consultants Ltd | Education | 2024 |
| Marina Express Ltd | Transport & logistics | 2023 |
| Marina Point Management Company Limited | Households | 1993 |
| Marina Supermarket Ltd | Wholesale & retail | 2023 |
| Marine Chartering Limited | Other services | 1994 |
| Marine Management Surveys Limited | Other services | 1996 |
| Mariners Court (Woodbridge) Limited | Real estate | 2001 |
| Marioraysa Ltd | Transport & logistics | 2021 |
| Maripetro Ltd | Transport & logistics | 2019 |
| Mariscot Wessex Ltd | Professional & scientific | 2025 |
| Maritime Haulage Limited | Extraterritorial | 2008 |
| Maritime West LP | | 2015 |
| Marius-Hristo Mihai Ltd | Transport & logistics | 2025 |
| Mark Barber Ltd | Construction | 2013 |
| Mark Bridges Carvers & Gilders Limited | Construction | 2003 |
| Mark Flewitt Therapies Limited | Arts & recreation | 2012 |
| Mark Goodwin Wealth Management Ltd | Financial services | 2014 |
| Mark Henson Ltd | Professional & scientific | 2023 |
| Mark Joy Plumbing & Heating Limited | Construction | 2004 |
| Mark Meadows Limited | Construction | 2018 |
| Mark Walters Ltd | Professional & scientific | 2014 |
| Markely Services Limited | Information & communication | 2025 |
| Market Impact LP | | 2024 |
| Market Marketing Ltd | Professional & scientific | 2017 |
| Market Scope Europe Limited | Information & communication | 2003 |
| Marketize Systems Limited | Information & communication | 2026 |
| Marks & Mann Stowmarket Ltd | Real estate | 2017 |
| Marks & Mann Ltd | Real estate | 2013 |
| Marks & Mann Martlesham Ltd | Real estate | 2016 |
| Marlborough Bowls Club Limited | Real estate | 1924 |
| Marlborough Road (Ipswich) Maintenance Company Limited | Real estate | 1987 |
| Marluc Construct Limited | Construction | 2011 |
| Marluc Group Limited | Financial services | 2020 |
| Marluc Limited | Professional & scientific | 2012 |
| Marlyn Commercial Vehicles Ltd | Wholesale & retail | 2015 |
| Marmalade IS Ltd | Professional & scientific | 2010 |
| Marmaris Kebab House Ltd | Hospitality | 2025 |
| Maro 4 Ltd | Professional & scientific | 2017 |
| Maroknyi Szekely Ltd | Transport & logistics | 2019 |
| Maronmark Limited | Professional & scientific | 2002 |
| Marr Procurement Limited | Professional & scientific | 2014 |
| Marriott Holdings Limited | Wholesale & retail | 2000 |
| Marriott Motor Group Limited | Wholesale & retail | 1999 |
| Mars Capital Group Ltd | Professional & scientific | 2024 |
| Marsam Supplies Ltd | Other services | 2009 |
| Marsden Rentals Limited | Real estate | 2017 |
| Marsena Ltd | Construction | 2024 |
| Marsford Limited | Extraterritorial | 1993 |
| Marsh Accountancy Limited | Professional & scientific | 2009 |
| Marshall GAS Ltd | Construction | 2017 |
| Marshall GAS Management Limited | Construction | 2019 |
| Marshall Stores Limited | Wholesale & retail | 2024 |
| Marst Agri Ltd | Professional & scientific | 2022 |
| Martello Solutions Limited | Administrative & support | 2025 |
| Martimedia Ltd | Information & communication | 2008 |
| Martin & Black Limited | Construction | 2020 |
| Martin & Newby Ltd | Real estate | 2019 |
| Martin Fenn Limited | Manufacturing | 2003 |
| Martin Freeman Trustee Company Ltd | Extraterritorial | 2023 |
| Martin James Bell Associates Limited | Administrative & support | 2024 |
| Martin Jarvis Ltd | Wholesale & retail | 2014 |
| Martine J Butcher Limited | Professional & scientific | 2011 |
| Martins & Jones Cleaning Ltd | Administrative & support | 2024 |