| Masser Construction Limited | Construction | 1995 |
| Masser Holdings Ltd | Construction | 1998 |
| Mastercars (Tempsford) Limited | Wholesale & retail | 2010 |
| Masterleisure (Tempsford) Limited | Wholesale & retail | 2011 |
| Mastermind Academics Limited | Education | 2019 |
| Mastery in Maths LLP | | 2026 |
| Material Maintenance Ltd | Construction | 2022 |
| Material Management Ltd | Construction | 2019 |
| Materials Movement Limited | Construction | 1993 |
| Mathematical Dexterity Limited | Education | 2022 |
| Mathematical Proficiency LLP | | 2025 |
| Mathletes of Crouch END LLP | | 2025 |
| Mathletes of Muswell Hill LLP | | 2025 |
| Maths Tutors - Barnet LLP | | 2023 |
| Maths Tutors - Whetstone LLP | | 2021 |
| Mathsmastersuk Ltd | Education | 2025 |
| Matrack Limited | Professional & scientific | 2013 |
| Matt Alexander Health Ltd | Health & social care | 2023 |
| Matt Sanderman Design Limited | Arts & recreation | 2007 |
| Matthew Gullick Real Estate Ltd | Real estate | 2025 |
| Maven International Limited | Manufacturing | 2002 |
| Mawdorbeena Limited | Real estate | 2025 |
| MAX HRM UK Ltd | Wholesale & retail | 2018 |
| MAX Mcmurdo Ltd | Professional & scientific | 2019 |
| MAX Power Rail Ltd | Professional & scientific | 2014 |
| Maximus Leonis Capital Limited | Financial services | 2026 |
| Maybe Limited | Professional & scientific | 2024 |
| Mayday Strategy Ltd | Professional & scientific | 2026 |
| Maydencroft Farm Limited | Extraterritorial | 2010 |
| Maydencroft Gardens Freehold Limited | Households | 2020 |
| Maydencroft Limited | Administrative & support | 2007 |
| Maytree Demolition Limited | Construction | 2018 |
| Maytree Groundworks Limited | Construction | 2018 |
| Maytree Group Herts Limited | Construction | 2018 |
| Maytree Landscapes Limited | Administrative & support | 2015 |
| MB Diamond Drilling Ltd | Construction | 2010 |
| MB Simm Limited | Professional & scientific | 2016 |
| MB T/A Services Limited | Administrative & support | 2021 |
| MBM Scaffolding Limited | Construction | 2021 |
| Mbmk Holdings Limited | Financial services | 2015 |
| MBS Recovery Limited | Administrative & support | 2022 |
| MCA Topco Limited | Professional & scientific | 2024 |
| MCB Partners Limited | Real estate | 2023 |
| Mccabe-Enterprises Limited | Administrative & support | 2017 |
| Mcdonnell Bozas Limited | Administrative & support | 2015 |
| Mcdowall Productions Limited | Arts & recreation | 2017 |
| Mcgrath Civils Ltd | Construction | 2015 |
| Mcjade Properties Limited | Real estate | 2012 |
| MCK Brickwork Limited | Construction | 2021 |
| Mckenzie and Sons Limited | Manufacturing | 2019 |
| Mclaren Resourcing Services Limited | Administrative & support | 2025 |
| MCS - Aware | Other services | 2011 |
| MCS Markets Ltd | Administrative & support | 2011 |
| MD Vision Limited | Wholesale & retail | 2020 |
| MDR Digital Limited | Other services | 2023 |
| MDR Investments Hitchin Ltd | Real estate | 2022 |
| MDS Electrical Contracting Limited | Construction | 2011 |
| MDW Contractors Limited | Water & waste | 2014 |
| MDX Data Ltd | Information & communication | 2025 |
| MDY Auto Ltd | Wholesale & retail | 2025 |
| ME Amore Beauty Ltd | Other services | 2024 |
| ME3L Ltd | Real estate | 2024 |
| Mead House Enterprises Ltd | Extraterritorial | 2013 |
| Mead Maintenance Limited | Construction | 2021 |
| Measurand Ltd | Professional & scientific | 2011 |
| Media Bear Limited | Manufacturing | 2018 |
| Mediandarte Limited | Construction | 2023 |
| Medic 22 Limited | Health & social care | 2023 |
| Meduck Associates Ltd | Information & communication | 2016 |
| Meea Embossers Limited | Manufacturing | 2020 |
| Mefisl Limited | Professional & scientific | 2005 |
| Megan Nicola Ltd | Education | 2023 |
| Mehra Property Investments Ltd | Real estate | 2024 |
| Melanie Lewis Bespoke Estate Agent Ltd | Real estate | 2022 |
| Melb Holdings Limited | Financial services | 2020 |
| Meliora Health Limited | Health & social care | 2020 |
| Melita Advisory Limited | Professional & scientific | 2016 |
| Mercury Intelligence Ltd | Professional & scientific | 2025 |
| Merge Sports Limited | Professional & scientific | 2015 |
| Meridian Co-Living Ltd | Real estate | 2023 |
| Meridian Contractor Services Limited | Administrative & support | 2014 |
| Meridian Elemental Health Ltd | Health & social care | 2026 |
| Meridian Health Alliance Ltd | Administrative & support | 2026 |
| Meridion Ltd | Information & communication | 2026 |
| Merryfields Sun Street Limited | Real estate | 2025 |
| Mery Beauty Limited | Other services | 2014 |
| Meta Build UK Limited | Real estate | 2022 |
| Meta Building Services Limited | Real estate | 2022 |
| Metabolic, Obesity & Bariatric Healthcare Ltd | Health & social care | 2024 |
| Metal Manufactured Products Ltd | Manufacturing | 2021 |
| Methi Hitchin Ltd | Hospitality | 2024 |
| Method Compliance and Assurance Ltd | Professional & scientific | 2018 |
| Method Cyber Security Ltd | Professional & scientific | 2018 |
| Method Functional Safety Ltd | Professional & scientific | 2013 |
| Method Process Safety Ltd | Professional & scientific | 2018 |
| Method Safety and Security Ltd | Professional & scientific | 2018 |
| Method Software Solutions Ltd | Professional & scientific | 2019 |
| Metriway Ltd | Information & communication | 2026 |
| Mews Plant Hire Ltd | Manufacturing | 2023 |
| MF Independent Consultant Limited | Health & social care | 2024 |