| M&G Home Improvement Ltd | Construction | 2018 |
| M&I Drylining Ltd | Construction | 2018 |
| M&S Accountancy Limited | Professional & scientific | 2023 |
| M&Sm Limited Ltd | Transport & logistics | 2014 |
| M. Barton Limited | Professional & scientific | 2009 |
| M. S. Thanisa Limited | Professional & scientific | 2019 |
| M.A.N Electrical Ltd | Construction | 2023 |
| M.C.M.Contractor Building Ltd | Construction | 2015 |
| M.K Express Car Wash Ltd | Administrative & support | 2025 |
| M.K Joinery 1 Limited | Manufacturing | 2025 |
| M.R Healthcare Limited | Other services | 2024 |
| M.R. Property Management (Bucks) Limited | Real estate | 2018 |
| M.T. Safety Inspections Ltd | Professional & scientific | 2018 |
| M1 Transporter Ltd | Wholesale & retail | 2020 |
| M2S Transport Limited | Transport & logistics | 2017 |
| M3R Maintenance Limited | Construction | 2016 |
| M77 Marketing Ltd | Information & communication | 2019 |
| M8 Transport Ltd | Transport & logistics | 2023 |
| MA Truck Training Limited | Education | 2014 |
| MAA Consultants Ltd | Professional & scientific | 2023 |
| MAA Land Limited | Administrative & support | 2015 |
| MAC Facilities Limited | Administrative & support | 2026 |
| MAC Haulage Services Ltd | Transport & logistics | 2025 |
| Mach Ville Limited | Real estate | 2024 |
| Maciej Technical Solution Ltd | Professional & scientific | 2025 |
| Macleod Company Limited | Wholesale & retail | 2022 |
| Macnaughton Limited | Administrative & support | 2002 |
| Macsin Trans Ltd | Transport & logistics | 2018 |
| Madarasz and Hudson Limited | Arts & recreation | 2025 |
| MADEBY3D Ltd | Wholesale & retail | 2024 |
| Madforce Engineering Limited | Manufacturing | 2017 |
| Madtech Electrical Ltd | Construction | 2020 |
| Magic Deco Ltd | Construction | 2022 |
| Magical Castle Place Car Wash Limited | Administrative & support | 2025 |
| Magna Five Ltd | Wholesale & retail | 2021 |
| Magnetic Media Group Ltd | Information & communication | 2024 |
| Magnum Services Limited | Administrative & support | 2024 |
| Magnus Effect Ltd. | Wholesale & retail | 2011 |
| Magoo's Bar Limited | Extraterritorial | 2013 |
| Magoos Ltd | Hospitality | 2003 |
| Magpie Nest Finance Limited | Financial services | 2015 |
| Mahmood Property Holdings Ltd | Real estate | 2026 |
| Mahmoodzay Ltd | Wholesale & retail | 2024 |
| Mahr Limited | Extraterritorial | 2017 |
| MAID2CLEAN (Bedfordshire) Ltd | Other services | 2012 |
| MAID2CLEAN (Watford) Ltd | Administrative & support | 2014 |
| Maidenhead Green Limited | Real estate | 2020 |
| Maindset Ltd | Information & communication | 2026 |
| Maintenance MEN London Limited | Construction | 2022 |
| Maira Express Limited | Hospitality | 2018 |
| Maison Johàn Limited | Real estate | 2025 |
| Maitree Travels & Transport Limited | Transport & logistics | 2025 |
| Maiz Property Ltd | Real estate | 2018 |
| Majestic Homes Care and Support Ltd | Health & social care | 2025 |
| Majestic Miracle Ltd | Hospitality | 2024 |
| Majestic Mood Ltd | Hospitality | 2024 |
| Makai Logistic Ltd | Transport & logistics | 2021 |
| Make a Memory Motorhome Hire Ltd | Administrative & support | 2023 |
| Maken Associates Ltd | Transport & logistics | 2008 |
| Maketimeforyou Ltd | Other services | 2023 |
| Making Tax Doable Limited | Information & communication | 2025 |
| Maknoon Abayas Ltd | Wholesale & retail | 2024 |
| Malchemy Ltd | Professional & scientific | 2024 |
| Maliks Express Cookham Ltd | Hospitality | 2019 |
| Maliks Express Limited | Hospitality | 2013 |
| Maliks of Marlow Limited | Hospitality | 2009 |
| Malton Hill Limited | Hospitality | 2017 |
| Malwa Transport Ltd | Transport & logistics | 2020 |
| MAM Consortium Limited | Real estate | 2020 |
| Mamoori Consulting Limited | Health & social care | 2026 |
| Mamun Ltd | Financial services | 2019 |
| Management Psychology Limited | Professional & scientific | 1994 |
| Managing Excellence Through Training Limited | Administrative & support | 2008 |
| Manchester Hotel Holdings Limited | Hospitality | 2023 |
| Mandrina Limited | Professional & scientific | 1994 |
| Manestyleuk Limited | Other services | 2026 |
| Mangia E Bevi Ltd | Wholesale & retail | 2020 |
| Mango Trucking Ltd | Transport & logistics | 2023 |
| Mangrove Plastering Ltd | Construction | 2024 |
| Manibus-Dei Limited | Manufacturing | 2026 |
| Manifox Ltd | Real estate | 2025 |
| Manjit Construction Ltd | Construction | 2019 |
| Manland Homes Limited | Financial services | 2020 |
| Manor Farm (Edlesborough) Management Company Limited | Households | 2015 |
| Manor Farm Grange Properties Limited | Construction | 2013 |
| Manor Specialist Resources Limited | Administrative & support | 2021 |
| Mansfield Advice Centre Ltd | Financial services | 2018 |
| Mantra App Ltd | Information & communication | 2025 |
| Manufacturing by Design EOT Limited | Administrative & support | 2025 |
| Manufacturing by Design Holdings Limited | Administrative & support | 2014 |
| Manzil Star Limited | Real estate | 2021 |
| Maplewood Property Investment Ltd | Real estate | 2019 |
| Maplewood Short Term Lets Ltd | Real estate | 2020 |
| Marble & Granite Surfaces Ltd | Construction | 2016 |
| Marble Designs Limited | Professional & scientific | 1987 |
| Marcin Carrier Limited | Transport & logistics | 2019 |
| Marden Hill Limited | Hospitality | 2024 |
| Mardoc Building Company Limited | Real estate | 2023 |
| Mardoc Developments Limited | Real estate | 2025 |
| Mardoc Properties Limited | Construction | 2012 |