| M.E.C Mortgages Limited | Professional & scientific | 2020 |
| M.J. Properties (Hove) Limited | Real estate | 1987 |
| M.L. Property Company Limited | Construction | 1984 |
| M.P.M. Specialist Solutions Limited | Other services | 1997 |
| M.S. Investments Limited | Extraterritorial | 1993 |
| M.S.G. (Marketing) Limited | Information & communication | 1985 |
| M.T.M. Units Limited | Manufacturing | 2000 |
| M.Y. Electrical Limited | Construction | 2017 |
| M/S Salib Property Ltd | Real estate | 2022 |
| M3 Technology Services Ltd | Wholesale & retail | 2026 |
| M8 Designs Ltd | Construction | 2024 |
| MA Lalji Dental Limited | Health & social care | 2024 |
| MA Partnership Services Limited | Professional & scientific | 2010 |
| Maban Ltd | Information & communication | 2013 |
| MAC Building Ltd | Construction | 2004 |
| MAC Project Ltd | Information & communication | 2008 |
| Macani Property Ltd | Manufacturing | 2025 |
| Macc Health Ltd | Health & social care | 2025 |
| Macconvilles Limited | Professional & scientific | 1998 |
| Machan Wealth Management Limited | Financial services | 2023 |
| Machin Group Limited | Construction | 2015 |
| Mackay Chambers Ltd | Professional & scientific | 2014 |
| Mackenzie Thorpe Limited | Arts & recreation | 1999 |
| Maclean Legal Search Limited | Other services | 2017 |
| Macleod LGV Services Ltd | Education | 2004 |
| Macleod Pinsent Care Conifer Ltd | Health & social care | 2005 |
| Macleod Pinsent Care Homes Ltd | Health & social care | 2007 |
| Macri Property Ltd | Real estate | 2025 |
| Macsareus Ltd | Other services | 2023 |
| Madame EAT Limited | Manufacturing | 2021 |
| Madame Jennifer Distillery Limited | Manufacturing | 2017 |
| Made by Magna Ltd | Manufacturing | 2020 |
| Made by Silk Ltd | Information & communication | 2011 |
| Made Minimal Ltd | Information & communication | 2015 |
| Madecourses Limited | Education | 2024 |
| Madeira Place Limited | Real estate | 2024 |
| Madoux Winfield Limited | Administrative & support | 2019 |
| Madproperty Limited | Real estate | 2024 |
| MAE TIC Kitchen Ltd | Hospitality | 2022 |
| Maestoso Music Co Limited | Wholesale & retail | 2002 |
| Maestro Motors Ltd | Wholesale & retail | 2020 |
| Magdalena Napora Limited | Health & social care | 2019 |
| Magical Design Ltd | Information & communication | 2019 |
| Magicowls Ltd | Information & communication | 2026 |
| Magictorch Ltd | Arts & recreation | 2002 |
| Magli Moves Limited | Other services | 2015 |
| Magnell Limited | Hospitality | 2016 |
| Magnetic North Music Ltd | Arts & recreation | 2012 |
| Magnolia Tree Day Nursery Limited | Health & social care | 2024 |
| Maguire & SON Construction Limited | Administrative & support | 2008 |
| Maguire & SON Holdings Ltd | Financial services | 2023 |
| Maguire & SON Properties Ltd | Construction | 2023 |
| Mahan Food & Drinks Limited | Manufacturing | 2017 |
| Mahan Foods Industries Limited | Manufacturing | 2018 |
| Mahan Foods Limited | Manufacturing | 2018 |
| Mahl Limited | Arts & recreation | 2018 |
| Maiacara Limited | Health & social care | 2017 |
| Maidstar Limited | Real estate | 1997 |
| Maison DES Fleurs Limited | Households | 2004 |
| Maison Roshi Limited | Manufacturing | 2022 |
| Maiz Engineering Ltd | Manufacturing | 2014 |
| Maiz Trading Ltd | Wholesale & retail | 2018 |
| MAK Premier Management Services Ltd | Hospitality | 2026 |
| Maka Sussex Ltd | Hospitality | 2023 |
| Makaruis Salib Limited | Health & social care | 2019 |
| Makaveli IT Services Ltd | Information & communication | 2013 |
| Makbal Consultant Ltd | Professional & scientific | 2017 |
| Makindo Learning Limited | Education | 2025 |
| Making KIN Ltd | Hospitality | 2025 |
| Making Sense of Retirement Limited | Administrative & support | 2010 |
| Mako Events Ltd | Hospitality | 2015 |
| Malcolm Finch Limited | Information & communication | 2008 |
| Maltpress Limited | Households | 1994 |
| Maltravers Road Ltd | Real estate | 2017 |
| Mamma Orange Ltd | Information & communication | 2021 |
| Mammoth Website Optimisation Limited | Administrative & support | 2024 |
| Mamursa Software & Consultancy Limited | Information & communication | 2022 |
| Management & Development (UK) Limited | Other services | 2008 |
| Management Elite Asset Ltd | Real estate | 2025 |
| Management Learning & Coaching Ltd | Professional & scientific | 2008 |
| Mango Pubs Ltd | Hospitality | 2024 |
| Mangotrout Ltd | Administrative & support | 2025 |
| Mangrove Break Limited | Real estate | 2013 |
| Mankind UK Limited | Health & social care | 2000 |
| Mannstone Holdings Ltd | Financial services | 2025 |
| Mannstone Properties Ltd | Real estate | 2025 |
| Manquos LLP | | 2011 |
| Mansell Mctaggart Hove Limited | Real estate | 2024 |
| Mansion Gardens Management Company Limited | Households | 1983 |
| Mantis Holdings Limited | Financial services | 2025 |
| Manton and Nash Construction Limited | Construction | 2023 |
| Manton and Nash Property Development Limited | Construction | 2024 |
| Manton Property Development Limited | Construction | 2025 |
| Manuka Design Limited | Construction | 2012 |
| Manuka Films Limited | Information & communication | 2006 |
| Manzi Consultancy Limited | Administrative & support | 2013 |
| Maple Building Company Limited | Construction | 2010 |
| Maple Grove "C" Management Company Limited | Households | 1989 |
| Maple Grove Freehold Company Limited | Households | 1998 |
| Maple Leaf Contractors Ltd | Construction | 2014 |