| Mobius Core Ltd | Financial services | 2025 |
| Modena Engineering Limited | Wholesale & retail | 2019 |
| Modern British Canteen Limited | Hospitality | 2004 |
| Modern Heritage Construction Ltd | Construction | 2019 |
| Modern Print Studio Ltd | Information & communication | 2019 |
| Modern Tennis International Limited | Arts & recreation | 2008 |
| Modnbass Ltd | Wholesale & retail | 2025 |
| Modomo Limited | Construction | 2020 |
| Moghrabi Holdings Limited | Financial services | 2021 |
| Mojo Leigh Limited | Hospitality | 2017 |
| Molendinum Limited | Hospitality | 2020 |
| Molly's Grooming Limited | Other services | 2022 |
| Momentum Carers Ltd | Health & social care | 2025 |
| Momentum Hypnotherapy Limited | Health & social care | 2024 |
| Monarch Construction Projects Ltd | Construction | 2026 |
| Monatree Ltd | Construction | 2024 |
| Mondello Family Ltd | Administrative & support | 2019 |
| Monometer Holdings Limited | Manufacturing | 1930 |
| Monster Inflatables Franchising Group Limited | Professional & scientific | 2023 |
| Monster Inflatables Limited | Arts & recreation | 2019 |
| Mont Ford Recruitment Ltd | Administrative & support | 2019 |
| Mont Ford Topco Limited | Financial services | 2023 |
| Montaini Automotive Ltd | Wholesale & retail | 2002 |
| Montbeau Properties Limited | Real estate | 2016 |
| Montedare Consulting Ltd | Other services | 2023 |
| Moombix UK Limited | Information & communication | 2024 |
| Moonfleet (Cornwall) Limited | Hospitality | 2022 |
| Mooros Chigwell Ltd | Hospitality | 2018 |
| Moraldo GAS & Heating Ltd | Construction | 2020 |
| More Electrical Limited | Administrative & support | 2014 |
| Morelight Glass Design Limited | Construction | 2017 |
| Morgan Heath & Associates Limited | Professional & scientific | 2013 |
| Mortgage and Co UK Ltd | Financial services | 2024 |
| Mortgage City Spain Limited | Financial services | 2002 |
| Mortgagehawk Limited | Financial services | 2025 |
| Morus Nigra Limited | Administrative & support | 2006 |
| Morveen & Co. Limited | Information & communication | 1973 |
| Mossgen Ltd | Professional & scientific | 2015 |
| Mostar Barbers Limited | Other services | 2025 |
| Mota Broker Services Ltd | Financial services | 2024 |
| Mota Risk Solutions Limited | Information & communication | 2025 |
| Motor-On South East Limited | Wholesale & retail | 2020 |
| Mountdale Building & Construction Labour Ltd | Construction | 2023 |
| Moussa Online PT Limited | Arts & recreation | 2024 |
| Move Mortgages Essex Limited | Financial services | 2023 |
| Moyderwell Capital Ltd | Professional & scientific | 2024 |
| Moylans Design Ltd | Wholesale & retail | 2012 |
| MPH Boilers Limited | Construction | 2018 |
| MPT Draughting Services Ltd | Professional & scientific | 2023 |
| MR Garden Limited | Administrative & support | 2023 |
| MR Hira Limited | Health & social care | 2021 |
| MR SB Limited 121 Ltd | Administrative & support | 2025 |
| MR White & Friends Ltd | Information & communication | 2017 |
| MR Wolf Kids Ltd | Wholesale & retail | 2020 |
| MR. Smith's Cleaning Services Ltd | Administrative & support | 2022 |
| Mrbp Property Limited | Real estate | 2024 |
| MRS Barnes' Bakery Limited | Hospitality | 2022 |
| MRS Magical Ears Ltd | Manufacturing | 2026 |
| MSG PM Limited | Professional & scientific | 2021 |
| MSL Property Lettings Ltd | Real estate | 2024 |
| Mudco Eco Limited | Wholesale & retail | 2025 |
| Mum&Sons Estates Ltd | Real estate | 2025 |
| Mummy's Money Ltd | Financial services | 2026 |
| Munroe Maintenance Limited | Other services | 2008 |
| Murray House Maintenance Company Limited | Other services | 1990 |
| Murtess Limited | Real estate | 2015 |
| Muscle HUT Medway Limited | Arts & recreation | 2022 |
| Muscle HUT Milton Keynes Ltd | Arts & recreation | 2025 |
| Muscle HUT Prop Co Limited | Real estate | 2024 |
| Musclehut Herne Bay Ltd | Arts & recreation | 2023 |
| Musclehut Hold Co Limited | Financial services | 2024 |
| Musclehut Topco Limited | Financial services | 2024 |
| Music Gateway Ltd | Information & communication | 2011 |
| Music Gateway Publishing Limited | Information & communication | 2018 |
| Mussard & Co. Wealth Management Limited | Financial services | 2010 |
| Must Impress Ltd | Information & communication | 2011 |
| Mustardgreenenergy Ltd | Financial services | 2025 |
| Mvbs (UK) Limited | Professional & scientific | 2014 |
| MVR Direct Limited | Wholesale & retail | 2020 |
| MWD Construction Ltd | Construction | 2013 |
| MWM Engineering Solutions Ltd | Professional & scientific | 2025 |
| MWS Supplies Ltd | Hospitality | 2022 |
| My Casa Away Limited | Households | 2019 |
| My Drum Ltd | Real estate | 2023 |
| My MED VA Ltd | Administrative & support | 2017 |
| My PAD Holdings Limited | Real estate | 2023 |
| My Type on Paper Ltd | Other services | 2023 |
| My Virtually Limited | Information & communication | 2009 |
| MY2 Ventures Limited | Wholesale & retail | 2024 |
| Mystycroft Property Limited | Real estate | 2025 |
| Mytown Southend CIC | Administrative & support | 2010 |
| N Bailey Partnership Limited | Construction | 2013 |
| N Briggs Limited | Professional & scientific | 2017 |
| N L Electrical & Maintenance Limited | Construction | 2019 |
| N R Powell Developments Limited | Construction | 1986 |
| N R Powell Holdings Limited | Financial services | 2016 |
| N R Powell Limited | Construction | 2016 |
| N&S Noblemart Ltd | Wholesale & retail | 2026 |
| N-Act Theatre in Schools CIC | Education | 2021 |
| N.A.S. Bhatti Healthcare Ltd | Health & social care | 2021 |