| Meridian Assurance Limited | Construction | 2022 |
| Meridian Energy Consulting Ltd | Construction | 2015 |
| Meridian Legacy Developments Ltd | Real estate | 2025 |
| Merlette Marketing Ltd | Professional & scientific | 2013 |
| Meron Scott Ltd | Information & communication | 2016 |
| Metamorphis Films Ltd | Information & communication | 2016 |
| Metamorphosis GP Limited | Other services | 2005 |
| Metis Strategic Advisory Ltd | Professional & scientific | 2024 |
| MGA Asset Management Limited | Other services | 2011 |
| MGF Portfolio Ltd | Professional & scientific | 2025 |
| MHR Renovations Ltd | Construction | 2026 |
| Michael L Ambrose Limited | Professional & scientific | 2011 |
| Michaela Henshaw Limited | Professional & scientific | 2015 |
| Middle Space Ltd | Professional & scientific | 2025 |
| Middlebrook Property Limited | Real estate | 2021 |
| Midha Property Consultancy Limited | Real estate | 2022 |
| Midha Property Group Limited | Real estate | 2019 |
| Midnight Sky Events Ltd | Administrative & support | 2022 |
| Milldale Services Ltd | Wholesale & retail | 2019 |
| Millie's Property Investment Ltd | Real estate | 2023 |
| Milton Court Harpenden Limited | Households | 1956 |
| Milton Road Management Company Limited(The) | Households | 1982 |
| Mind & Byte Limited | Information & communication | 2018 |
| Mind Bust Limited | Information & communication | 2023 |
| Mini WIN Games Limited | Information & communication | 2026 |
| Miniburst Limited | Professional & scientific | 2000 |
| Minpark Limited | Extraterritorial | 1964 |
| Mint Architecture & Design Ltd | Professional & scientific | 2023 |
| Mintec Quality Systems Ltd | Manufacturing | 2006 |
| Mira Developments Limited | Construction | 2006 |
| Missouri Belle Limited | Real estate | 2023 |
| Mixdown Management Limited | Arts & recreation | 2016 |
| MJM Ventures Ltd | Professional & scientific | 2021 |
| MLC Consultancy Services Ltd | Professional & scientific | 2026 |
| MLP Research Limited | Wholesale & retail | 2026 |
| Model Prisoner the Movie Limited | Information & communication | 2019 |
| Modernland Limited | Real estate | 2012 |
| Modish Amour Limited | Wholesale & retail | 2017 |
| Modlight Electrical Ltd | Construction | 2021 |
| Modular Urban Spaces Ltd | Manufacturing | 2024 |
| Mokai Ltd | Information & communication | 2024 |
| Moloney Capital Investments Limited | Construction | 2014 |
| Moloney Capital Limited | Construction | 2014 |
| Moloney Capital Management Services Limited | Households | 2019 |
| Moloney Group Limited | Administrative & support | 2021 |
| Momo Marketing Limited | Extraterritorial | 2015 |
| MON IP Ltd | Information & communication | 2020 |
| Mondial Consultancy Limited | Professional & scientific | 2008 |
| Monetizely Media Ltd | Information & communication | 2024 |
| Money Mechanix Ltd | Financial services | 2017 |
| Monkey Music Limited | Education | 1996 |
| Monteagle Partnership Ltd | Professional & scientific | 2025 |
| Montgomery & Cunnington Limited | Professional & scientific | 2017 |
| Moondarra EV Ltd | Wholesale & retail | 2025 |
| Moonfire Consultancy Limited | Other services | 2020 |
| Moonlit Consultancy Ltd | Wholesale & retail | 2020 |
| Moorhouse Care Home Limited | Health & social care | 2026 |
| Moox Design Studio Ltd | Professional & scientific | 2019 |
| Moraine Advisors Ltd | Administrative & support | 2021 |
| Morissa Limited | Hospitality | 2018 |
| Morris Environmental (UK) Limited | Water & waste | 2014 |
| Mortimer Health Ltd | Health & social care | 2022 |
| Morton Green Mortgages Limited | Financial services | 2020 |
| Mossette Ltd | Financial services | 2018 |
| Mother and Baby Coach Limited | Wholesale & retail | 2020 |
| Mother Tongue Specialist Limited | Professional & scientific | 2025 |
| Motion International Group UK Limited | Construction | 2018 |
| Motivo Consulting Ltd | Arts & recreation | 2023 |
| Motoring Research Trustees Limited | Administrative & support | 2019 |
| Motorwise of Brentford Limited | Transport & logistics | 2006 |
| Mould Matters Limited | Administrative & support | 2024 |
| Mount Lodge Investments Limited | Professional & scientific | 2024 |
| Mount View Dance School Ltd | Professional & scientific | 2002 |
| Movency Limited | Information & communication | 2005 |
| Moynihan Training and Coaching Limited | Other services | 2012 |
| MP Auto Detailing Ltd | Wholesale & retail | 2019 |
| MP Auto Group Limited | Wholesale & retail | 2024 |
| MR Bluetail Limited | Professional & scientific | 2026 |
| MRP MRJ Ltd | Wholesale & retail | 2025 |
| MRS Gray Limited | Professional & scientific | 2022 |
| MS Recycling Ltd | Wholesale & retail | 2023 |
| MSG Broadcast Ltd | Information & communication | 2020 |
| MSH Data Centers Limited | Information & communication | 2025 |
| MSH Planning Limited | Information & communication | 2010 |
| Mulberry Door Holdings Ltd | Real estate | 2025 |
| Multa Group Limited | Agriculture & fishing | 2021 |
| Multimedia Computer Training Limited | Information & communication | 1994 |
| Mum's Guide to Limited | Information & communication | 2012 |
| Murat Property Maintenance & Developments Ltd | Construction | 2020 |
| Murray Business Consultancy Ltd | Public administration | 2020 |
| Murray Recycling Limited | Transport & logistics | 2019 |
| Murton Court Management Limited | Households | 1975 |
| Muse Management Consultants Ltd | Administrative & support | 2023 |
| Musicale Limited | Other services | 2018 |
| MV Acoustic Design Ltd | Professional & scientific | 2025 |
| MVS IT Solutions and Consulting Ltd | Information & communication | 2018 |
| MVT Management Ltd | Information & communication | 2023 |
| MW Distribution and Logistics Ltd | Wholesale & retail | 2024 |
| My African Hairitage Style (Mahs) Ltd | Wholesale & retail | 2016 |
| My Alpha Limited | Information & communication | 2026 |