| Mcmanus Electrical Contractor Ltd | Construction | 2023 |
| Mcneil Wealth Management Limited | Financial services | 2026 |
| Mcpartland Planning Ltd | Professional & scientific | 2020 |
| Mcrm Operations Ltd | Information & communication | 2013 |
| MCS Contract Cleaning Limited | Administrative & support | 2000 |
| MCS Waste Group Limited | Water & waste | 2023 |
| MD Spice Co. Ltd | Wholesale & retail | 2021 |
| Mdmv Studio Ltd | Professional & scientific | 2024 |
| MDN Prospect Ltd | Transport & logistics | 2015 |
| MDR Services Ltd | Information & communication | 2024 |
| ME and RAE Limited | Wholesale & retail | 2023 |
| ME Tomorrow Limited | Professional & scientific | 2015 |
| Meadow Close Limited | Administrative & support | 2000 |
| Meadow Holdings Limited | Real estate | 1955 |
| Meadow Road Maisonettes Limited | Other services | 2003 |
| Meadow Vale (Wheathampstead) Management Company Limited | Real estate | 2011 |
| Meadowbourne Properties Ltd | Real estate | 2025 |
| Meads Consulting Ltd | Professional & scientific | 2024 |
| Meakin Consultancy Ltd | Professional & scientific | 2021 |
| Meat Emporium (ST Albans) Ltd | Wholesale & retail | 2022 |
| Meaty Millie Online Coaching Ltd | Education | 2024 |
| Mechanical Building Services Consultancy Limited | Construction | 2021 |
| Mechanical Handling Assured Limited | Wholesale & retail | 2013 |
| MED Deli Ltd | Wholesale & retail | 2022 |
| MED Meat Shop Ltd | Wholesale & retail | 2024 |
| Med-Choice MR Ltd | Professional & scientific | 2012 |
| Mede Holidays Limited | Hospitality | 2021 |
| Media Finance AI Ltd | Information & communication | 2025 |
| Media Plus Ltd | Professional & scientific | 2023 |
| Media Powerhouse Group Ltd. | Administrative & support | 2008 |
| Media Powerhouse Limited | Professional & scientific | 2007 |
| Mediazone First LLP | | 2023 |
| Medic's ARE US Ltd | Health & social care | 2025 |
| Medicare Europe Ltd | Manufacturing | 2011 |
| Medicare Solutions Direct Limited | Health & social care | 2017 |
| Medina Butchers Ltd | Wholesale & retail | 2019 |
| Meditation Courses Limited | Health & social care | 2020 |
| Medlycase Ltd | Education | 2026 |
| Medready Training Ltd | Education | 2026 |
| Medrescue 24 Ltd | Health & social care | 2019 |
| Meek Care Limited | Health & social care | 2024 |
| Meet ME at Mabel’S Ltd | Arts & recreation | 2025 |
| Meganhair Limited | Other services | 2025 |
| MEI Marketing Ltd | Other services | 2025 |
| Meiilin Ltd | Hospitality | 2026 |
| Meijer Seed Potato Limited | Administrative & support | 2007 |
| Mekatronics Ltd | Manufacturing | 2020 |
| Melburyfi Limited | Arts & recreation | 2024 |
| Meldog Ltd | Professional & scientific | 2018 |
| Mellcrest Limited | Other services | 1988 |
| Melon Patch Investment Limited | Wholesale & retail | 2020 |
| Melon Properties Limited | Real estate | 2023 |
| Melrose Advisors Ltd | Professional & scientific | 2022 |
| Melyor Property Services Ltd | Real estate | 2023 |
| Memcs Ltd | Construction | 2026 |
| Mendes Hopkins Ltd | Professional & scientific | 2023 |
| MENSCH24 Ltd | Real estate | 2025 |
| Menspire Education International Limited | Education | 2017 |
| Menspire Education Limited | Education | 2015 |
| Menspire Education Products Limited | Professional & scientific | 2017 |
| Menspire Global Distribution Ltd | Other services | 2021 |
| Menspire Harpenden Limited | Other services | 2018 |
| Menspire Harpenden Products Limited | Other services | 2020 |
| Menspire Investments Limited | Real estate | 2020 |
| Menspire Limited | Other services | 2012 |
| Menspire Products Limited | Other services | 2017 |
| Menspire Radlett Limited | Other services | 2017 |
| Menspire Radlett Products Limited | Other services | 2019 |
| Menta-Estate Ltd | Real estate | 2023 |
| Mentored by Menspire Global Ltd | Education | 2026 |
| Meola Group Limited | Financial services | 2016 |
| Mercantile Bonds and Surety Limited | Extraterritorial | 2025 |
| Mercer & Hole Audit LLP | | 2022 |
| Mercer & Hole LLP | | 2022 |
| Merchant Supplies Distribution Limited | Wholesale & retail | 2025 |
| Mercury 22 Nominees Limited | Financial services | 2022 |
| Merit Computer Systems Limited | Other services | 2002 |
| Merits Health Products UK Ltd | Health & social care | 2023 |
| Merlin Digital Consulting Limited | Information & communication | 2016 |
| Merlot Mews Management Company Limited | Households | 2008 |
| Merricat Pictures Ltd | Information & communication | 2022 |
| Merrishaw Limited | Professional & scientific | 2019 |
| Merryhill Kitchens Ltd | Hospitality | 2026 |
| Merryweather Ventures Limited | Financial services | 2020 |
| Mesh Theatre Co. Ltd | Arts & recreation | 2019 |
| Meshcloud UK Ltd | Information & communication | 2025 |
| MET Medical Ambulance Limited | Health & social care | 2026 |
| Metaco LLP | | 2006 |
| Metalayer Limited | Information & communication | 2013 |
| Metalwood Fencing (Contracts) Limited | Construction | 1988 |
| Metaman Ltd | Professional & scientific | 2025 |
| Metaspace Limited | Professional & scientific | 2001 |
| Metaxops UK Limited | Information & communication | 2022 |
| Metcalfepubs Ltd | Hospitality | 2025 |
| Methodize Counselling & Therapy Services Ltd | Health & social care | 2025 |
| Metis Diagnostics Ltd | Health & social care | 2024 |
| Metomic Ltd | Information & communication | 2018 |
| Metropolis Safety Solutions Limited Liability Partnership | | 2008 |
| Metrotest Scientific Ltd | Financial services | 2018 |
| Mfrnet Ltd | Wholesale & retail | 2023 |