| Mastermag Limited | Professional & scientific | 1979 |
| Matatika Limited | Information & communication | 2012 |
| Matt & IVY Ltd | Construction | 2022 |
| Matt Brown Limited | Administrative & support | 2011 |
| Matt Buckley Photo Limited | Professional & scientific | 2021 |
| Matt Farrell Properties Ltd | Real estate | 2024 |
| Matthew Mojsak Ltd | Health & social care | 2016 |
| Matthews Architectural Consulting Ltd | Professional & scientific | 2024 |
| Matties Gardening Limited | Administrative & support | 2017 |
| Maughold Ventures Limited | Professional & scientific | 2025 |
| Mavencroft Limited | Construction | 2021 |
| Maxamal Limited | Arts & recreation | 2015 |
| Mayfair Property Limited | Real estate | 2003 |
| Mazco Recruitment Ltd | Administrative & support | 2023 |
| MC Inspire LLP | | 2020 |
| Mccarthy Visual Limited | Information & communication | 2023 |
| Mcdonaghs Solicitors (UK) LLP | | 2016 |
| Mcdonaghs Solicitors Limited | Professional & scientific | 2016 |
| Mcewen Property Services Limited | Real estate | 2013 |
| Mcgrellis Building Limited | Construction | 2016 |
| Mckean Medical Imaging Limited | Health & social care | 2020 |
| Mckinley Pharma Limited | Wholesale & retail | 2017 |
| Mcwigg Ltd | Households | 2020 |
| MDI Investments Ltd | Financial services | 2023 |
| Mead Hearing Solutions Limited | Extraterritorial | 2016 |
| Medpa Ltd | Administrative & support | 2026 |
| Melbe Home UK Ltd | Wholesale & retail | 2024 |
| Melboc Ltd | Professional & scientific | 2025 |
| Melies Home Ltd | Real estate | 2026 |
| Melodyville Limited | Real estate | 1981 |
| Meridien Capital Management Limited | Administrative & support | 2014 |
| Merling Management Limited | Real estate | 2004 |
| Merritt International Limited | Professional & scientific | 2025 |
| Message House Holdings Limited | Financial services | 2024 |
| Message House Ltd | Professional & scientific | 2013 |
| Metal Detection Limited | Professional & scientific | 2007 |
| Metals Detox Ltd | Information & communication | 2011 |
| Metric Carpets Limited | Wholesale & retail | 1977 |
| Metro Properties (99) Limited | Real estate | 2002 |
| MH Pallets Limited | Wholesale & retail | 2012 |
| MH Property Management Services Limited | Real estate | 2019 |
| Mialto Technology Solutions Ltd | Information & communication | 2024 |
| Mighty Sports Limited | Education | 2017 |
| Mike Lovatt's Brass Pack Limited | Arts & recreation | 2023 |
| Mike Peacock Limited | Construction | 2005 |
| Miles Mitchell Ltd | Information & communication | 2025 |
| Millbrook (Caistor) Management Company Limited | Households | 2019 |
| Mint Blue Ltd | Administrative & support | 2012 |
| Mirala Limited | Wholesale & retail | 1981 |
| Mirrored Magic Limited | Professional & scientific | 2023 |
| Mitris Estates Ltd | Real estate | 2026 |
| MJ Thornton Ltd | Construction | 2019 |
| Mjms Ltd | Professional & scientific | 2004 |
| MJR Racing Ltd | Arts & recreation | 2024 |
| ML Roofing Solutions Ltd | Construction | 2022 |
| Moatyi Ltd | Real estate | 2022 |
| Moma Group Limited | Professional & scientific | 2023 |
| Monarch Cycles Limited | Wholesale & retail | 2026 |
| Moneta Bridge Ltd | Professional & scientific | 2025 |
| Moniker Limited | Professional & scientific | 2003 |
| Monkey Puzzle Day Nurseries (Dormant 1) Limited | Health & social care | 2021 |
| Monkey Puzzle Day Nurseries (Dormant 2) Limited | Health & social care | 2021 |
| Monkey Puzzle Day Nurseries (Ealing) Limited | Education | 2007 |
| Monkey Puzzle Day Nurseries (East Sheen) Limited | Health & social care | 2021 |
| Monkey Puzzle Day Nurseries (Guildford) Limited | Health & social care | 2021 |
| Monkey Puzzle Day Nurseries (Lightwater) Ltd | Health & social care | 2021 |
| Monkey Puzzle Day Nurseries (Loughton) Limited | Education | 2021 |
| Monkey Puzzle Day Nurseries (Thanos) Limited | Education | 2015 |
| Monkey Puzzle Day Nurseries (West Kensington) Limited | Health & social care | 2021 |
| Monkey Puzzle Day Nurseries (Woodford Green) Limited | Education | 2021 |
| Monkey Puzzle Day Nurseries Limited | Professional & scientific | 2004 |
| Monkey Puzzle Group Holdings Limited | Financial services | 2021 |
| Monkey Wizard Films Ltd | Information & communication | 2024 |
| Montague Angel & Company Limited | Professional & scientific | 2014 |
| Moo-Coco Ltd | Wholesale & retail | 2020 |
| Moonfleet Consulting Limited | Information & communication | 2025 |
| Moredoc Solutions Ltd | Administrative & support | 2025 |
| Moreton Street Limited | Real estate | 2004 |
| MOT Models Limited | Administrative & support | 2005 |
| Mountford Financial Management Ltd | Information & communication | 2012 |
| Movement Performance Physiotherapy Ltd | Health & social care | 2024 |
| Movie Props Ltd | Information & communication | 2021 |
| Moyvale Investments Limited | Administrative & support | 1997 |
| Moyvale Properties Limited | Construction | 1976 |
| Moyvale Property Care Ltd | Construction | 2003 |
| MP Partnership Limited | Administrative & support | 2009 |
| Mpdn Holdings Limited | Extraterritorial | 2011 |
| Mpdn Limited | Health & social care | 2021 |
| Mpdn Properties Limited | Real estate | 2015 |
| MPF Landscapes Ltd | Administrative & support | 2025 |
| Mpng Limited | Health & social care | 2021 |
| MR Randle Ltd | Administrative & support | 2017 |
| MR Smith Entertainment Ltd | Arts & recreation | 2017 |
| Mr&Mrs Kushch Ltd | Real estate | 2025 |
| Mreps Trustees (NO 1) Limited | Professional & scientific | 2002 |
| Mrmsown Ltd | Construction | 2025 |
| MSB Advisors Limited | Professional & scientific | 2014 |
| MSB RE (67 Station Road) Limited | Real estate | 2023 |
| MSB RE (Wootton Bridge) Limited | Real estate | 2023 |
| Msm-Advisory Limited | Other services | 2025 |