| Maris Meadow Management Company Limited | Households | 2021 |
| Maritime Content Ltd. | Information & communication | 2000 |
| Mark Butler Associates Limited | Other services | 1991 |
| Mark G O'Connor Limited | Professional & scientific | 2014 |
| Mark Roberts Audio Limited | Manufacturing | 2021 |
| Mark Scott Constructions Limited | Construction | 1974 |
| Mark Yates Ltd | Hospitality | 2009 |
| Market Edge Consulting Limited | Professional & scientific | 2000 |
| Marketing Skills Trust Limited | Education | 1991 |
| Marks & Tilt Ltd. | Wholesale & retail | 2016 |
| Markyate Car Wash Limited | Administrative & support | 2023 |
| Markyate Headquarters Ltd | Health & social care | 2024 |
| Markyate Indian Spice Ltd | Hospitality | 2026 |
| Markyate Precision Machining Co Limited | Professional & scientific | 2001 |
| Marlboro' Motors Limited | Wholesale & retail | 1995 |
| Marlborough Opportunities Ltd | Administrative & support | 2019 |
| Marley’S Wheathampstead Limited | Hospitality | 2022 |
| Marli Ltd | Professional & scientific | 2025 |
| Marloo UK Limited | Information & communication | 2026 |
| Marmitous Trading Limited | Manufacturing | 2026 |
| Marnek Ltd | Real estate | 2010 |
| Marra Builders Ltd | Construction | 2023 |
| Married in Sicily Ltd | Administrative & support | 2026 |
| Marsh Street North Limited | Real estate | 2026 |
| Marshal Spice Ltd | Hospitality | 2023 |
| Martin and Martin Finance Ltd | Information & communication | 2019 |
| Martin Collins Entertainment Limited | Arts & recreation | 2012 |
| Martin Lavelle Leadership in Education Consultancy Ltd | Professional & scientific | 2026 |
| Martin Noble Associates Ltd | Professional & scientific | 2024 |
| Martin Property Holdings Ltd | Real estate | 2018 |
| Martin Ramsey Ltd | Professional & scientific | 2023 |
| Martin Reed Creative Ltd | Professional & scientific | 2018 |
| Martin Smith Commercial Motors LLP | | 2023 |
| Martin Sprocket & Gear UK Limited | Manufacturing | 2020 |
| Martock Design Limited | Professional & scientific | 1977 |
| Martyn Guess & Company Limited | Construction | 2003 |
| MAS Investments London Ltd | Real estate | 2021 |
| Mash Money Limited | Financial services | 2024 |
| Masherob Limited | Health & social care | 2015 |
| Masons Group Ltd | Construction | 2021 |
| Massabalala Holdings PTY Ltd | Wholesale & retail | 2022 |
| Mastercraft Renovations Limited | Construction | 2025 |
| Masterpiece Consulting Limited | Real estate | 2007 |
| Masterpiece Education and Training Limited | Education | 2020 |
| Masterpiece Education CIC | Education | 2021 |
| Masters Close Investments Limited | Real estate | 2020 |
| Masters Games Ltd | Wholesale & retail | 2001 |
| Mastery Media Ltd | Information & communication | 2024 |
| Match Centre UK Limited | Information & communication | 2025 |
| Mater DEI Management Company Limited | Households | 1984 |
| Material Bidder Limited | Information & communication | 2017 |
| Mathorize Ltd | Information & communication | 2023 |
| Matlucha Management Limited | Administrative & support | 2013 |
| Matrix Software Limited | Information & communication | 2019 |
| Matrix Technology Consulting Limited | Information & communication | 2012 |
| Matter + Mindset Ltd | Professional & scientific | 2025 |
| Matters Property Ltd | Households | 2016 |
| Matthew & Daisy Home Limited | Real estate | 2021 |
| Matthew FOX Driving School Ltd | Education | 2024 |
| Matthew Holman Consulting Limited | Professional & scientific | 2021 |
| Matthew Investments Limited | Construction | 1978 |
| Mattrex Limited | Professional & scientific | 2015 |
| Maturo Limited | Information & communication | 2026 |
| Maughan Consulting Associates Limited | Administrative & support | 2010 |
| Mawqwid Ltd | Information & communication | 2023 |
| MAX Whitby Services Ltd | Information & communication | 2026 |
| Maxitone Limited | Information & communication | 2022 |
| Maxymus Floor Care Ltd | Administrative & support | 2015 |
| MAY Pigeons Limited | Agriculture & fishing | 2025 |
| MAY Pigeons Racing & Breeding Centre Limited | Agriculture & fishing | 2021 |
| Maya Brands Limited | Professional & scientific | 2024 |
| Mayers Ltd | Wholesale & retail | 2001 |
| Mayfair Place Ltd | Professional & scientific | 2023 |
| Maylands Practice Limited | Health & social care | 2025 |
| Mayloe Construction Limited | Construction | 2022 |
| Maywins Group Limited | Financial services | 2025 |
| Maywins Motorsport Limited | Wholesale & retail | 2025 |
| Mazovia Group Limited | Professional & scientific | 2024 |
| Maztin Ltd | Wholesale & retail | 2023 |
| MB Landscaping & Construction Ltd | Administrative & support | 2025 |
| MB Plumbing Services Ltd | Construction | 2024 |
| MB Property Investments Limited | Households | 2001 |
| MBA Placements Ltd | Real estate | 2025 |
| Mbjfireprotection Ltd | Public administration | 2025 |
| Mbosuk Ltd | Wholesale & retail | 2022 |
| MC Drinks Limited | Wholesale & retail | 2010 |
| MC2 Ecommerce Ltd | Professional & scientific | 2026 |
| Mcauliffe Holdings Limited | Financial services | 2020 |
| MCC Property Consultants Ltd | Real estate | 2026 |
| Mccall Advisory Limited | Professional & scientific | 2025 |
| Mccartney Associates Limited | Professional & scientific | 2007 |
| MCD Security Herts Limited | Administrative & support | 2012 |
| Mcdi Logistics Ltd | Transport & logistics | 2019 |
| Mceherts Ltd | Manufacturing | 2020 |
| Mcentric Technology Limited | Information & communication | 2000 |
| Mcgaw&Co Ltd | Professional & scientific | 2026 |
| Mckelvy's Limited | Professional & scientific | 2017 |
| Mckenovations Limited | Real estate | 2022 |
| Mcleod Group Services Ltd | Professional & scientific | 2026 |
| MCM Housing Ltd | Real estate | 2021 |