| Metal Roofing Ltd | Construction | 2023 |
| Metalab Ltd | Information & communication | 2002 |
| Methusellah Limited | Hospitality | 2023 |
| Metrix Ltd | Information & communication | 2026 |
| Mexicanas Limited | Hospitality | 2018 |
| MF Imaging Limited | Health & social care | 2023 |
| Mfms Properties LLP | | 2023 |
| MFS Portfolio Limited | Professional & scientific | 2005 |
| MG Management Consulting Limited | Professional & scientific | 2010 |
| MG Retreat Ltd | Real estate | 2025 |
| MG Social Limited | Information & communication | 2017 |
| MG Tiles Limited | Wholesale & retail | 2000 |
| Mgfilms Ltd | Information & communication | 2026 |
| Mggm Logistics Ltd | Transport & logistics | 2025 |
| MGS Groundworks Limited | Construction | 2020 |
| MGS Ventures Ltd | Real estate | 2025 |
| MH Interiors Ltd | Manufacturing | 2025 |
| MHM (Sussex) Ltd | Construction | 2025 |
| MI Culture Ltd | Wholesale & retail | 2025 |
| MIA Regan Ltd | Other services | 2020 |
| Miabo Ltd | Real estate | 2019 |
| Miabo Property Ltd | Real estate | 2018 |
| Mibrand Limited | Manufacturing | 2002 |
| MIC MMC Ltd | Construction | 2017 |
| Michael Lordan Associates Ltd | Construction | 2017 |
| Michael Seare Limited | Health & social care | 2012 |
| Michael Toller Ltd | Other services | 2017 |
| Michael Wenman Holdings Limited | Financial services | 2025 |
| Michael Wenman Limited | Construction | 1979 |
| Michelle Leighton Buying Consultancy Limited | Professional & scientific | 2019 |
| Mick Perrin Worldwide Holdings Limited | Professional & scientific | 2002 |
| Mick Perrin Worldwide Limited | Arts & recreation | 2017 |
| Mick Perrin Worldwide Management Limited | Arts & recreation | 2017 |
| Micro Shaping Ltd | Administrative & support | 2006 |
| Midascode Ltd | Administrative & support | 2006 |
| Middlechild Productions Limited | Administrative & support | 2005 |
| Middleton Bespoke (Sussex) Limited | Construction | 2009 |
| Mifie Properties Ltd | Real estate | 2019 |
| Mihara Donegan Productions Ltd | Arts & recreation | 2008 |
| Mikat Audio Ltd | Information & communication | 2016 |
| Mike Corby Courts Limited | Arts & recreation | 1972 |
| Mike Corby Management Limited | Arts & recreation | 1988 |
| Mike Corby Properties Limited | Financial services | 2006 |
| Mike Hammond Motors Limited | Wholesale & retail | 2004 |
| Mike Ring Ltd | Wholesale & retail | 2025 |
| Mikhael Investments Limited | Financial services | 2014 |
| Mile Oak Flats Limited | Households | 2025 |
| Mile Oak Road Developments Limited | Construction | 2023 |
| Milkshake Blue Limited | Professional & scientific | 2020 |
| Mill Hill Catering Ltd | Hospitality | 2025 |
| Millard Installations Electrical Services Ltd | Construction | 2025 |
| Millbeech Limited | Administrative & support | 1997 |
| Millenio Ltd | Professional & scientific | 2020 |
| Miller Parris Solicitors LLP | | 2020 |
| Millgreen Ltd | Real estate | 2022 |
| Millgrove Construction Limited | Construction | 2009 |
| Millhayes Investments Limited | Real estate | 2018 |
| Millie MAC Makeup Ltd | Administrative & support | 2020 |
| Milliken Ltd | Professional & scientific | 2024 |
| Milo Pickle Limited | Real estate | 2025 |
| Milo TV Limited | Information & communication | 2006 |
| Milspecdata Ltd | Information & communication | 2026 |
| Milton Court (Worthing) Management Limited | Households | 1977 |
| Mimic Media Ltd | Information & communication | 2001 |
| Minaires Limited | Financial services | 2020 |
| Minaires Management Limited | Real estate | 2016 |
| Minarah Residences Ltd | Real estate | 2024 |
| Mind and Health Limited | Health & social care | 2014 |
| Mindcloud Limited | Other services | 2009 |
| Mindful Research in AI Limited | Professional & scientific | 2025 |
| Mindfulness Sussex Ltd | Health & social care | 2012 |
| Mindset-Thetrainingclub Ltd | Arts & recreation | 2026 |
| Mindsetup Ltd | Professional & scientific | 2019 |
| Mindsight Healthcare Limited | Health & social care | 2021 |
| Mine IT Limited | Wholesale & retail | 2021 |
| Mini Golf South East LLP | | 2009 |
| Ministry of Vision Limited | Extraterritorial | 1996 |
| Minmxd Limited | Professional & scientific | 2024 |
| Mint Acorn Property Limited | Real estate | 2024 |
| Mint Design Associates Ltd | Professional & scientific | 2000 |
| Mint Motor Car Services Ltd | Wholesale & retail | 2024 |
| Mintaxiom Limited | Professional & scientific | 2014 |
| Mintertainment Holdings Limited | Professional & scientific | 2017 |
| Mintertainment Limited | Arts & recreation | 2014 |
| Mirage Holdings Ltd | Information & communication | 2024 |
| Miranda Birch Media Ltd | Professional & scientific | 2017 |
| Miras Installations Limited | Wholesale & retail | 2010 |
| Mirfield Contracting Limited | Administrative & support | 2011 |
| MISHAJASON1 Ltd | Health & social care | 2021 |
| Miss B Childcare Limited | Health & social care | 2025 |
| Miss JAY GEE Ltd | Administrative & support | 2016 |
| Miss Moles Flower Emporium Limited | Wholesale & retail | 2018 |
| Missgreedys Limited | Professional & scientific | 2021 |
| Missing Pieces HR Limited | Professional & scientific | 2008 |
| Mission Masters Limited | Administrative & support | 2002 |
| Mister Guitar Limited | Wholesale & retail | 2009 |
| Mitchells Consulting Limited | Professional & scientific | 2024 |
| Mith Marketing Consultants Ltd | Administrative & support | 2025 |
| Mixal Ltd | Health & social care | 2015 |
| Mixxa Limited | Arts & recreation | 2009 |