| M&M Singh Catering Ltd | Hospitality | 2025 |
| M&M Singh Constructions Limited | Construction | 2025 |
| M&N Tank Property Ltd | Real estate | 2024 |
| M-Tech Systems Limited | Information & communication | 2003 |
| M-Verse Group Limited | Professional & scientific | 2023 |
| M. & S. Print Services Limited | Manufacturing | 1998 |
| M. Comley Limited | Professional & scientific | 2004 |
| M.B Building and Property Maintenance Limited | Construction | 2025 |
| M.G.B. Holdings Limited | Financial services | 1985 |
| M.J.B Farriery Limited | Administrative & support | 2010 |
| M.N.A. Consultants Limited | Health & social care | 2006 |
| M.S. Cornford Plumbing & Heating Limited | Construction | 2014 |
| M2 Project Surveyors Limited | Professional & scientific | 2020 |
| M2 Property Investments Ltd | Real estate | 2022 |
| M21 Xavelha Latin Deli Ltd | Wholesale & retail | 2025 |
| M21 Xavelha Snack Bar Limited | Wholesale & retail | 2023 |
| MAC Broking Limited | Financial services | 2017 |
| Macan Holdings Limited | Real estate | 2016 |
| Machen Oak Ltd | Financial services | 2025 |
| Mackay Group Holdings Limited | Financial services | 2024 |
| Maclee Construction Ltd | Construction | 2017 |
| Macmark Limited | Wholesale & retail | 2016 |
| Macremo Limited | Wholesale & retail | 2023 |
| Macro Drywall Limited | Construction | 2024 |
| Macsai Health Limited | Wholesale & retail | 2016 |
| MAD Group (Sussex) Limited | Other services | 2013 |
| Made at 82 Limited | Real estate | 2017 |
| Made Backwards Limited | Arts & recreation | 2003 |
| Madeira Windows & Conservatories Ltd | Construction | 2015 |
| Madibi Limited | Other services | 2014 |
| Madrideo Limited | Health & social care | 2013 |
| Maghsoudi Ltd | Financial services | 2020 |
| Magna Business Consulting Ltd | Administrative & support | 2016 |
| Magna Carta MOT & Repair Limited | Wholesale & retail | 2014 |
| Magnetic Business Academy Limited | Extraterritorial | 2019 |
| Magnetic Business Development Ltd | Professional & scientific | 2005 |
| Magsfactors Limited | Wholesale & retail | 2001 |
| Maguire Trading Co Ltd | Wholesale & retail | 2021 |
| Mahabba Network | Other services | 2012 |
| Mahayana Buddhism Foundation Limited | Other services | 2023 |
| Mahogany Developments Ltd | Real estate | 2025 |
| Mahony Actuarial Services Ltd | Professional & scientific | 2023 |
| MAI Accountants & Tax Advisers Limited | Professional & scientific | 2021 |
| Main Productions Limited | Information & communication | 2015 |
| Maintenance Free Products Limited | Professional & scientific | 1987 |
| Maison Damascene Limited | Health & social care | 2025 |
| Maison Privé Studios Ltd | Professional & scientific | 2022 |
| Maitech Group Ltd | Wholesale & retail | 2025 |
| Maize Developments Ltd | Construction | 2016 |
| Majestic Real Estate Eastbourne Ltd | Real estate | 2023 |
| Make a Difference Media Limited | Professional & scientific | 2015 |
| Make IT MAY Productions Ltd | Arts & recreation | 2025 |
| Make Life Click C.I.C. | Information & communication | 2020 |
| Make Way Music Ltd | Information & communication | 1988 |
| Makeover Box Limited | Other services | 2026 |
| Maki Dental Limited | Health & social care | 2022 |
| Malaks Medical Limited | Health & social care | 2010 |
| Malden Reach (Eastbourne) Limited | Households | 2004 |
| Malhas Medical Limited | Health & social care | 2020 |
| Malhi Traders Limited | Wholesale & retail | 2025 |
| Mallard Dental Services Limited | Health & social care | 2017 |
| Mallett Solutions Ltd | Information & communication | 2026 |
| Mama French Tacos Ltd | Hospitality | 2024 |
| Mana Gaming Ltd | Wholesale & retail | 2019 |
| MANAHATTA1609 Limited | Professional & scientific | 2014 |
| Mandeep Mitha Limited | Health & social care | 2014 |
| Mandeep Soorae Ltd | Health & social care | 2024 |
| Manga Rosa Ltd | Information & communication | 2024 |
| Manifold Mews Management Company Ltd | Construction | 2021 |
| Mankind SE CIC | Health & social care | 2021 |
| Manna Nexus CIC | Professional & scientific | 2024 |
| Manor Farm (UP Hatherley) Management Company Limited | Real estate | 2012 |
| Manoria Marketing Ltd | Administrative & support | 2026 |
| Manorparkward Ltd | Wholesale & retail | 2019 |
| Manson Davies Limited | Real estate | 2022 |
| Manta Proprietors Ltd | Real estate | 2018 |
| Manton Court Management Association Limited | Households | 1969 |
| Mantra Training Sussex Ltd | Education | 2023 |
| Manzano Roofing & Building Ltd | Construction | 2023 |
| MAP Acquisitions Limited | Real estate | 2014 |
| MAP Property (S.E.) LLP | | 2020 |
| Mapa Builders Ltd | Construction | 2025 |
| Maple Made Developments Limited | Construction | 2017 |
| Mapletoft Care Limited | Administrative & support | 2010 |
| Mapperley Dental Care Limited | Health & social care | 2019 |
| Mapperley Developments Limited | Real estate | 2023 |
| Maprise Limited | Real estate | 2002 |
| MAR & DAR Ltd | Hospitality | 2020 |
| MAR Interests Ltd | Real estate | 2017 |
| Marathon Convenience Store Ltd | Wholesale & retail | 2023 |
| Marble 2 Marvel Ltd | Manufacturing | 2003 |
| March Hare Management Limited | Professional & scientific | 2025 |
| Marchmont Commercial Services Limited | Professional & scientific | 2025 |
| Marchmont Developments Limited | Construction | 2017 |
| Marchmont Estates Limited | Real estate | 2003 |
| Marchmont Group LLP | | 2008 |
| Marchmont Ltd | Professional & scientific | 2025 |
| Marchmont Properties Limited | Real estate | 2009 |
| Marchmont Property Surveyors Limited | Professional & scientific | 2025 |
| Marchmont Surveyors Limited | Professional & scientific | 2013 |