| Marvel Access Limited | Construction | 2012 |
| Mary Barnicott Art and Design Limited | Manufacturing | 2015 |
| Masar Systems LLP | | 2018 |
| Mask Jsnl Limited | Real estate | 2021 |
| Masters Building Contractors Ltd | Construction | 2002 |
| Mathy Property Ltd | Real estate | 2026 |
| MATTHEW2707 Ltd | Information & communication | 2014 |
| Maxe.Ai Ltd | Financial services | 2021 |
| Maxiflow TM Ltd | Construction | 2024 |
| MAY Property Investments Ltd | Real estate | 2005 |
| Maya Pest Control Limited | Administrative & support | 2019 |
| Maycroft Solutions Limited | Professional & scientific | 2023 |
| Maze Building Services Ltd | Construction | 2018 |
| MB Loft Conversions Ltd | Construction | 2022 |
| Mcgrath Glazing Company Ltd | Construction | 2021 |
| MCI Motorcycles Ltd | Wholesale & retail | 2023 |
| Mckenna Electrical LLP | | 2014 |
| Mckinley Mills Limited | Professional & scientific | 2000 |
| Mcweald Limited | Construction | 2026 |
| Mdcp Ltd | Construction | 2026 |
| Meaningful Change Ltd | Professional & scientific | 2015 |
| Mechanical Ventilation Services Limited | Construction | 2025 |
| Mechanicus Ltd | Construction | 2025 |
| Mediately Limited | Extraterritorial | 2015 |
| Medical Professionals Academy Limited | Education | 2010 |
| MEDICARE365 Ltd | Wholesale & retail | 2020 |
| Medivision Limited | Professional & scientific | 2000 |
| Mega Scaffolding Ltd | Construction | 2008 |
| Melanie Jayne Ltd | Real estate | 2019 |
| Melbury Gardens Management Company Ltd | Households | 2008 |
| Melmo Shutters and Blinds Limited | Construction | 2022 |
| Membame Ltd | Information & communication | 2020 |
| Mercantile Investments Limited | Administrative & support | 2017 |
| Mercantile Investments SA Ltd | Real estate | 2024 |
| Mercantile Property Company Limited | Financial services | 2017 |
| Merceric Ltd | Education | 2020 |
| Meridian Water BTL 1 Limited | Real estate | 2018 |
| Merton Dental Wimbledon Ltd | Health & social care | 2024 |
| Merton Park Draughting Services Limited | Administrative & support | 2014 |
| Metadata Limited | Administrative & support | 1988 |
| Metraval Limited | Professional & scientific | 2025 |
| MFC Skips Limited | Water & waste | 2021 |
| MFE Commissioning Services Ltd | Construction | 2011 |
| MG Partnership Building Services Consulting Engineers Ltd | Professional & scientific | 2007 |
| Mgduk Limited | Professional & scientific | 2018 |
| Miakaa Ltd | Other services | 2020 |
| Michael Brown (Bakers) Limited | Real estate | 1975 |
| Michos & Co (Engineering) Limited | Professional & scientific | 2025 |
| Microtek Engineering Limited | Administrative & support | 2022 |
| Midnight Nocturna Limited | Construction | 2025 |
| Midsummer Associates Limited | Professional & scientific | 2026 |
| MIK ( PVT) Ltd | Transport & logistics | 2017 |
| Mike Birch Limited | Other services | 2016 |
| Mike Lilley Ltd | Professional & scientific | 2014 |
| Millbank Land and Properties Limited | Real estate | 2020 |
| Millbank Nursery Schools Limited | Education | 2013 |
| Millcroft Properties Limited | Real estate | 1996 |
| Millway Consulting Limited | Professional & scientific | 2022 |
| Mimi's Babywear Limited | Wholesale & retail | 2025 |
| Mimosa Residents Association (Woldingham) Limited | Households | 1974 |
| Mindflex Leadership Limited | Extraterritorial | 2017 |
| Minerva Property Developments (MPD) Limited | Real estate | 2018 |
| Minerva Tabletop Games Ltd | Wholesale & retail | 2021 |
| Mint Walk Ltd | Wholesale & retail | 2025 |
| Mission Group Services Limited | Administrative & support | 2024 |
| Mission Landscapes Limited | Administrative & support | 2025 |
| Miyad Group Limited | Real estate | 2025 |
| Mjbeducationalconsulting Ltd | Education | 2024 |
| MJH Structural Design Limited | Professional & scientific | 2019 |
| MJM Building & Maintenance Ltd | Construction | 2023 |
| MK and Co Accountants Limited | Professional & scientific | 2018 |
| ML Communications Ltd | Information & communication | 2006 |
| MLS Group Investments Ltd | Wholesale & retail | 2022 |
| MMG Fabrications Limited | Manufacturing | 2024 |
| Mobile EV Charging Ltd | Wholesale & retail | 2024 |
| MOD Solar Ltd | Construction | 2025 |
| Moissanite Property Investments Limited | Real estate | 2022 |
| Molpour Investments Ltd | Real estate | 2026 |
| Monochrome Homes Group Ltd | Real estate | 2020 |
| Montagna Installations Limited | Professional & scientific | 2023 |
| Moore Environmental Commissioning Limited | Construction | 1999 |
| Morgan Controls Solutions Ltd | Construction | 2011 |
| Moriati Media (U.K.) Limited | Administrative & support | 1991 |
| Morphew Creative Designs Ltd | Arts & recreation | 2015 |
| Moth Lighting Ltd | Other services | 2013 |
| Motherwell Group Ltd | Financial services | 2020 |
| Motherwell Properties Limited | Real estate | 2019 |
| Moving on Care Management NFP Limited | Hospitality | 2018 |
| MPG Electrical Services Limited | Construction | 2022 |
| MPM Mechanical Services Limited | Construction | 2016 |
| MR Tish Ltd | Information & communication | 2011 |
| MRM Early Year Consultancy Ltd | Education | 2012 |
| Mslar Limited | Professional & scientific | 2023 |
| MT Wells Building Limited | Administrative & support | 2013 |
| Multidimensional Ltd | Administrative & support | 2018 |
| Mumbles Homes Limited | Hospitality | 2025 |
| Munster Properties Ltd | Real estate | 2020 |
| Murphy Building Services Group Limited | Professional & scientific | 2007 |
| Murphy Building Services Holdings Ltd | Financial services | 2021 |
| Murphy Building Services Ltd | Construction | 1995 |