| Mannion Estimating Services Limited | Professional & scientific | 2025 |
| Manor Drive (Cuckfield) Limited | Households | 2016 |
| Mansell Mctaggart Haywards Heath Limited | Real estate | 2014 |
| Mansell Mctaggart Holdings Limited | Real estate | 2016 |
| Mansell Mctaggart Lettings Ltd | Real estate | 2008 |
| Mantell Park Consulting Ltd | Professional & scientific | 2020 |
| Many Waters Capital Limited | Professional & scientific | 2017 |
| Maple Fire Risk Reports Ltd | Professional & scientific | 2024 |
| Maple Storage (Shoreham) Limited | Transport & logistics | 2026 |
| March Fourth Limited | Hospitality | 2026 |
| March Race Engineering Limited | Extraterritorial | 2011 |
| Marcot Consulting Ltd | Professional & scientific | 2023 |
| Maria Greaves Limited | Administrative & support | 2012 |
| Mariam Enterprise Ltd | Wholesale & retail | 2024 |
| Marian Cameron Consultants Limited | Professional & scientific | 2014 |
| Marine Holdings Limited | Construction | 1956 |
| Marinesafe Campaign Limited | Professional & scientific | 2015 |
| Marinesafe Limited | Professional & scientific | 2015 |
| Marinesafe Products Limited | Manufacturing | 2015 |
| Mark Aberdour Consulting Limited | Information & communication | 2024 |
| Mark Antony Windows Limited | Construction | 2009 |
| Mark Preston Training Ltd | Health & social care | 2025 |
| Mark Reid Construction Planning Limited | Professional & scientific | 2009 |
| Mark Revill & Co Ltd | Real estate | 2013 |
| Mark Riminton & Associates Ltd | Professional & scientific | 2023 |
| Marker Ventures Limited | Real estate | 2021 |
| Markko Labs Limited | Information & communication | 2024 |
| Marks Media Limited | Arts & recreation | 2015 |
| Marriott Sinclair LLP | | 2012 |
| Marshall Publishing Consultancy Limited | Professional & scientific | 2015 |
| Marten Developments (Sussex) Limited | Construction | 1981 |
| Martinique Holdings Ltd | Other services | 2015 |
| Marwit Limited | Financial services | 2019 |
| Maré by Rafael Cagali Ltd | Hospitality | 2025 |
| MAS Design Limited | Information & communication | 2010 |
| MAS Networks Ltd | Information & communication | 2012 |
| MAS Technicae Group (International) Ltd | Information & communication | 2012 |
| Masa Grupo Limited | Hospitality | 2023 |
| Mascot Group (Southern) Ltd | Construction | 2023 |
| Mascot Roofing & Metals Limited | Construction | 2019 |
| Masindi Healthcare Limited | Health & social care | 2016 |
| Maskill Solutions Limited | Professional & scientific | 2015 |
| Masra Limited | Real estate | 1991 |
| Masscoms Ltd | Information & communication | 2026 |
| Master Key Simulations Limited | Arts & recreation | 2017 |
| Mathilda Rose Limited | Wholesale & retail | 2011 |
| Matthew David Collins Ltd | Professional & scientific | 2013 |
| Matthew Neiland Limited | Financial services | 2015 |
| Maudes Construction Limited | Construction | 2025 |
| Maven Ventures Limited | Administrative & support | 2011 |
| Maverick HC Ltd | Professional & scientific | 2025 |
| MAX Botanics Limited | Wholesale & retail | 2024 |
| MAX Court Management Company Limited | Administrative & support | 2013 |
| Maxcom Networks LLP | | 2005 |
| Mayenda Limited | Arts & recreation | 2012 |
| Mayfields (Bolney) Limited | Households | 1996 |
| Mayforte Limited | Professional & scientific | 2012 |
| MB Fitness Coaching Ltd | Other services | 2022 |
| Mbizo Limited | Professional & scientific | 2022 |
| MBM Events Limited | Arts & recreation | 2016 |
| MC Tree Surgery Ltd | Administrative & support | 2021 |
| Mccauley Consulting Limited | Professional & scientific | 2014 |
| Mcfadden Nichols Limited | Professional & scientific | 1989 |
| Mckenna Consulting Ltd | Professional & scientific | 2019 |
| MCLEOD35 Ltd | Professional & scientific | 2021 |
| MCM Design & Build Ltd | Construction | 2026 |
| MCM Digital Ltd | Professional & scientific | 2024 |
| MCR Media Solutions Ltd | Manufacturing | 2015 |
| MCW Bolney Limited | Wholesale & retail | 2017 |
| MD Smart System Ltd | Manufacturing | 2022 |
| MDR Automotive Limited | Wholesale & retail | 2017 |
| Meadowgreen Public Limited Company | Wholesale & retail | 1985 |
| Meaningful Spaces Limited | Real estate | 2017 |
| Mearsy Services Limited | Construction | 2017 |
| Meat Master 09' Ltd | Manufacturing | 2021 |
| Mechanical Project Solutions Limited | Professional & scientific | 2009 |
| MED 7 Ltd | Other services | 2022 |
| Medbelt Limited | Manufacturing | 2014 |
| Media Blocks Ltd | Information & communication | 2022 |
| Media Bookers Ltd | Information & communication | 2005 |
| Media Builders Limited | Other services | 2022 |
| Media City AI Ltd | Professional & scientific | 2025 |
| Mediaroo Limited | Financial services | 2017 |
| Mediating Works Limited | Professional & scientific | 2004 |
| Medical Device Development Limited | Health & social care | 2010 |
| Medical Device Treatment Limited | Health & social care | 2011 |
| MEDPHARM14 Limited | Wholesale & retail | 2025 |
| Medserv Accounting Limited | Professional & scientific | 2022 |
| Medserv Limited | Administrative & support | 2009 |
| Medusa Communications Ltd | Professional & scientific | 2024 |
| Meet the Need Ltd | Education | 2015 |
| Melbourne Management Services Limited | Professional & scientific | 2019 |
| Mellors Court Freehold Limited | Real estate | 2013 |
| Mellors Court RTM Company Limited | Real estate | 2016 |
| Merlin Pharma Consulting Ltd | Professional & scientific | 2019 |
| Merrieweathers Road Management Company Limited | Households | 1993 |
| Mesoscopic Detail Limited | Extraterritorial | 2011 |
| Messrs. F.J. Nicholas LLP | | 2008 |
| MET Solutions Ltd | Administrative & support | 2020 |
| Metanature Ltd | Professional & scientific | 2023 |