| Maggiekrof Limited | Health & social care | 2022 |
| Magmatic Games Ltd | Information & communication | 2011 |
| Magna CP UK Limited | Administrative & support | 2025 |
| Magnet Motos Ltd | Wholesale & retail | 2022 |
| Magnolia Therapy Centre Ltd | Health & social care | 2023 |
| Magnum ICC UK Ltd | Wholesale & retail | 2024 |
| Mahr Consulting Ltd | Information & communication | 2018 |
| Maid for London Limited | Administrative & support | 2023 |
| Main Management Limited | Wholesale & retail | 2022 |
| Maison & Mews Developments Ltd | Construction | 2025 |
| Maison Constructions Ltd | Construction | 2024 |
| Maison Decorum Limited | Wholesale & retail | 2024 |
| Maison London Property Limited | Real estate | 2025 |
| Majestic Fabrics Shepherds Bush Ltd | Wholesale & retail | 2023 |
| Major Solutions IT Consultancy Ltd | Information & communication | 2013 |
| Majorboost Ltd | Professional & scientific | 2014 |
| MAK Electrical Solutions Ltd | Construction | 2013 |
| Makai Yachts Ltd | Manufacturing | 2019 |
| Makasem Ltd | Information & communication | 2025 |
| Makran Ltd | Construction | 2025 |
| Malabunicorn Limited | Education | 2015 |
| Malakai Publishing Ltd | Information & communication | 2017 |
| Malden & Coombe Residents' Association | Households | 1946 |
| Malevich Ltd | Arts & recreation | 2021 |
| Mallacoota Limited | Financial services | 2019 |
| Mamaxo Ltd | Financial services | 2022 |
| Mandinee's Limited | Hospitality | 2023 |
| Maneland Holdings Limited | Financial services | 2025 |
| Maneland Properties Limited | Real estate | 2025 |
| Manger Digital Ltd | Information & communication | 2026 |
| Manna Pictures UK Limited | Information & communication | 2018 |
| Manna Spices Limited | Wholesale & retail | 2025 |
| Mannin Motorcycles Limited | Education | 2012 |
| Manor Oak Projects Ltd | Construction | 2026 |
| Manorway (Media) Group Ltd | Financial services | 2023 |
| Manston Gardens (Ramsgate) Management Company Ltd | Households | 2025 |
| Mansur & Sons Ltd | Real estate | 2024 |
| Manteq Properties Limited | Real estate | 2018 |
| Manuman Creative Ltd | Information & communication | 2018 |
| Manyfield Limited | Construction | 1983 |
| Mapv Ltd | Real estate | 2017 |
| Mar&Fer Ltd | Wholesale & retail | 2023 |
| Mara Career Service Ltd | Administrative & support | 2024 |
| Mara Reporting Ltd | Health & social care | 2025 |
| Marc Paul Motors Limited | Wholesale & retail | 1997 |
| Marclab Limited | Health & social care | 2024 |
| Marcus Mcculloch Ltd | Professional & scientific | 2024 |
| Marechal HR Consulting Ltd | Professional & scientific | 2021 |
| Maresfield Real Estate Ltd | Households | 2012 |
| Margareth Ainley Limited | Administrative & support | 2015 |
| Margin (Group) Limited | Professional & scientific | 2013 |
| Margin Art & Media PLC | Administrative & support | 2024 |
| Marguerite Murdoch Architects Ltd | Professional & scientific | 2022 |
| Maria Grasso Design Associates Limited | Real estate | 1991 |
| Maria's Case Management Ltd | Health & social care | 2021 |
| Marina Place Limited | Households | 2010 |
| Marina Place Riverside Limited | Real estate | 2022 |
| Marinatto Ltd | Manufacturing | 2021 |
| Marine Care Limited | Wholesale & retail | 2004 |
| Mariners Compass Ltd | Information & communication | 2021 |
| Marita Services Ltd | Professional & scientific | 2025 |
| Maritz UK Ltd | Professional & scientific | 2020 |
| Marke Creative Merchandise Ltd | Wholesale & retail | 1977 |
| Market Corporation Limited | Wholesale & retail | 2020 |
| Marketarc Limited | Wholesale & retail | 2025 |
| Marlborough Consortium Limited | Professional & scientific | 2006 |
| Marlborough Gardens(East)(Surbiton)Residents Association Limited | Households | 1962 |
| Marlin House Limited | Other services | 1997 |
| Marlow Roofing Specialists Ltd | Construction | 2024 |
| Marmar Creative Ltd | Professional & scientific | 2006 |
| Marnane Consulting Limited | Professional & scientific | 2016 |
| MARNAV5 (UK) Ltd | Administrative & support | 2016 |
| Marray's Creative Studio Ltd | Professional & scientific | 2023 |
| Marsbros Ltd | Hospitality | 2019 |
| Marshfield Bakery Limited | Manufacturing | 2005 |
| Marspire Ltd | Administrative & support | 2017 |
| Marta M. Pires Limited | Professional & scientific | 2011 |
| Martin Alexandra Ltd | Extraterritorial | 2012 |
| Martin Ashley LLP | | 2012 |
| Martin Bidco Limited | Financial services | 2020 |
| Martin COX Advertising Ltd | Professional & scientific | 2023 |
| Martin Topco Limited | Financial services | 2020 |
| Martin Williamson Ltd | Professional & scientific | 2024 |
| Martin's PCM Ltd | Construction | 2020 |
| Martyn Bailey Limited | Wholesale & retail | 2005 |
| Maruti Food Ltd | Hospitality | 2026 |
| Marvat Limited | Health & social care | 2023 |
| Marvel Wealth Limited | Real estate | 2020 |
| Marwood Ventures Limited | Professional & scientific | 2025 |
| Mary ANN Weeks Wellbeing Limited | Other services | 2014 |
| Mary Cong Ltd | Professional & scientific | 2019 |
| Maryland Estates Limited | Construction | 1993 |
| Marymount International School | Education | 2005 |
| Marymount School Enterprises Limited | Education | 2011 |
| Marziale Limited | Administrative & support | 2011 |
| Masa Management Property Services Limited | Real estate | 2023 |
| Masa Management Services Ltd | Construction | 2020 |
| Masaltech Ltd. | Information & communication | 2018 |
| Masaya Beauty Ltd | Other services | 2019 |
| Mash Media Group Limited | Information & communication | 2002 |