| Manoir 17 Limited | Real estate | 2023 |
| Manor Farm Barns (East Dean) Limited | Households | 2010 |
| Manor Farm Close Residents Association Limited | Households | 1981 |
| Manor Franklin Limited | Professional & scientific | 2015 |
| Manor Residents Limited | Households | 1992 |
| Manorwood Construction Limited | Construction | 2015 |
| Mansion Acquisitions Limited | Construction | 1995 |
| Mansion Assets Limited | Construction | 2024 |
| Mansion Developments Holdings Limited | Construction | 2022 |
| Mansion Developments Limited | Construction | 2001 |
| Manta RAY Growth Limited | Financial services | 2026 |
| Manta RAY Holdings Limited | Other services | 2021 |
| Mantasmart Limited | Hospitality | 1988 |
| Many Many Clothes Ltd | Wholesale & retail | 2025 |
| Many Soft Ltd | Information & communication | 2019 |
| MAP Cars CHI Ltd | Transport & logistics | 2024 |
| MAP Enterprises Limited | Hospitality | 2020 |
| Mapleleaf Barn Limited | Real estate | 2006 |
| Mapleleaf Ventures Limited | Wholesale & retail | 2006 |
| MAR Girgis Limited | Hospitality | 2013 |
| Marber Limited | Professional & scientific | 2001 |
| Marble Mayne Recruitment Ltd | Administrative & support | 2018 |
| March Baby Creative Ltd | Information & communication | 2025 |
| Marchwood Building Ltd | Construction | 2026 |
| Marchwood IFA Limited | Financial services | 2009 |
| Marco Polo Intercontinental Limited | Wholesale & retail | 2012 |
| Marden Contracting Limited | Administrative & support | 2011 |
| Mardenair Limited | Arts & recreation | 1976 |
| Marianne Topham Limited | Arts & recreation | 2006 |
| Marida Hydrographic Limited | Administrative & support | 2008 |
| Marilyns Cakes Ltd | Wholesale & retail | 2013 |
| Marina Marbella (U.K.) Limited | Wholesale & retail | 1992 |
| Marine Boat Carpets Limited | Construction | 2025 |
| Marine Competitions Ltd | Other services | 2026 |
| Marine Data Ltd | Professional & scientific | 2009 |
| Marine Gardens (Selsey) Management Limited | Real estate | 1978 |
| Marine Mesh Services Limited | Agriculture & fishing | 2015 |
| Mariposa Crescent Limited | Financial services | 2025 |
| Marisa Heath Consultancy Limited | Professional & scientific | 2022 |
| Maritime & Country Ltd | Other services | 2017 |
| Maritime Atlantic Limited | Real estate | 1994 |
| Mark Laurence Design Ltd | Professional & scientific | 2013 |
| Mark Taylor RR & B Ltd | Wholesale & retail | 2014 |
| Marketing Academy Group Ltd | Professional & scientific | 2023 |
| Marketing Teacher Limited | Administrative & support | 2006 |
| Marketscan Data Solutions Ltd | Information & communication | 2025 |
| Markham Furniture & Joinery Limited | Manufacturing | 2001 |
| Markops Limited | Professional & scientific | 2013 |
| Marl Meadows Limited | Administrative & support | 2024 |
| Marsh Barns Management Limited | Real estate | 1988 |
| Marshall Keen Limited | Professional & scientific | 2002 |
| Marshalls Farm Limited | Agriculture & fishing | 2015 |
| Marston & Jones Engineering Limited | Manufacturing | 2015 |
| Martell Accountancy Services Limited | Professional & scientific | 2014 |
| Martell Construction Services Limited | Construction | 2012 |
| Martin Parrott Services Limited | Information & communication | 2000 |
| Martin's Antiques Limited | Wholesale & retail | 2013 |
| Martlett Management Limited | Real estate | 2011 |
| Masham Investments Ltd | Administrative & support | 2017 |
| Masilla Suarez Ltd | Health & social care | 2021 |
| Maslowski Ltd | Manufacturing | 2017 |
| Mason Financial Planning Limited | Financial services | 2021 |
| Masson Appointments Limited | Administrative & support | 2003 |
| Mastbold Limited | Extraterritorial | 1989 |
| Master Key Holdings Limited | Professional & scientific | 2016 |
| Master Spark Limited | Construction | 2019 |
| Masters Brickwork & Stonemasonry Ltd | Construction | 2018 |
| Materna UK Ltd | Wholesale & retail | 2020 |
| Mathew & Thomas Ltd | Hospitality | 2022 |
| Matt Chilton Television Ltd | Information & communication | 2014 |
| Matt Smith Ceramics Ltd | Education | 2023 |
| Mattblackbarn CIC | Education | 2023 |
| Matterhorn Management Limited | Administrative & support | 2008 |
| Matthew Ennever Services Ltd | Construction | 2018 |
| Matthew Hancock Ltd | Professional & scientific | 2014 |
| Matthew Walker Media Ltd | Information & communication | 2012 |
| MAV Properties (Southern) Limited | Real estate | 2025 |
| Maverick Group Holdings Ltd | Financial services | 2023 |
| Maverick Partners Ltd | Information & communication | 2020 |
| Maximize Your Potential Limited | Education | 2002 |
| Maximum Airport Cars Ltd | Transport & logistics | 2023 |
| Maximus Accountancy Services Limited | Professional & scientific | 2013 |
| Maximus Groundworks Ltd | Construction | 2018 |
| Maxis Menswear Limited | Wholesale & retail | 2024 |
| Maxwell Media Ltd | Professional & scientific | 2026 |
| MAY Film and Media Solutions Ltd | Manufacturing | 2021 |
| Maybour Logistics Global Limited | Transport & logistics | 1998 |
| Mayes Plaster Magic Ltd | Construction | 2025 |
| Mayfair House (Worthing) Management Limited | Households | 2013 |
| Mayfair ST. James Limited | Construction | 1998 |
| Mayhead Valuation Services Ltd | Administrative & support | 2017 |
| Mayhew Investments Limited | Real estate | 1962 |
| Mays Direct Ltd | Construction | 2025 |
| Mays Scaffolding Company Ltd | Construction | 2021 |
| Mays Web Design Ltd | Construction | 2011 |
| Maywick Properties Ltd | Real estate | 2021 |
| Mazby Limited | Real estate | 2023 |
| Mazetime Ltd. | Information & communication | 1998 |
| MB Seafood Limited | Hospitality | 2014 |
| Mbcs Architectural Services Ltd | Professional & scientific | 2018 |