| Martin Robson ARC Ltd | Financial services | 2014 |
| Martin Thwaites Consulting Limited | Information & communication | 2019 |
| Martin White Associates Limited | Professional & scientific | 2005 |
| Martin's Car Services Limited | Wholesale & retail | 2019 |
| Martindale TM Ltd | Construction | 2024 |
| Martins Chocolatier Limited | Manufacturing | 2009 |
| Martrl Limited | Real estate | 2024 |
| Martyn Coulter Ltd | Education | 2018 |
| Martz World Ltd | Professional & scientific | 2026 |
| Marula Consulting Limited | Transport & logistics | 2012 |
| Marulatec Limited | Professional & scientific | 2014 |
| Maruthi Consulting Limited | Health & social care | 2018 |
| Maruti Realty Limited | Real estate | 2024 |
| Marv Technologies Limited | Information & communication | 2023 |
| Marvel Coaching Ltd | Administrative & support | 2023 |
| Marvellous Training Solutions Limited | Administrative & support | 2002 |
| Marvick Ltd | Wholesale & retail | 2025 |
| Marvio KB Ltd | Information & communication | 2026 |
| Mary Hare Hearing Centres LLP | | 2007 |
| Maryamdulla Limited | Health & social care | 2012 |
| Maryland (Woburn) Management Company Limited | Households | 2012 |
| Maryland Apartments Limited | Households | 2017 |
| Marylebone Gardens (Milton Keynes) Management Company Limited | Households | 1999 |
| MAS Engineering Services Ltd | Professional & scientific | 2026 |
| MAS Installations MK Ltd | Construction | 2024 |
| MAS Resourcing Limited | Administrative & support | 2018 |
| Masa Essentials Ltd | Wholesale & retail | 2020 |
| Masai Construction Services Ltd | Administrative & support | 2025 |
| Masajora Healthcare Services Limited | Health & social care | 2020 |
| Mascass Ltd | Hospitality | 2019 |
| Masha Allaha Ltd | Wholesale & retail | 2020 |
| Mashlaw Track Limited | Extraterritorial | 2017 |
| Masi Manimba Ltd | Real estate | 2025 |
| Masiso Ltd | Transport & logistics | 2022 |
| Maslin Building Services Ltd | Professional & scientific | 2009 |
| Mason & Fifth Limited | Construction | 2017 |
| Mason & Fifth TIB Limited | Real estate | 2020 |
| Mason Enterprises Limited | Real estate | 2002 |
| Mason Residences UK Limited | Real estate | 2021 |
| Masons Estate Agent Ltd | Real estate | 2023 |
| Masons Investments LLP | | 2011 |
| Masons Luton Limited | Real estate | 2010 |
| Masons Property Experts Limited | Real estate | 2019 |
| Massage Therapy by OLA Ltd | Health & social care | 2025 |
| Massanc Pictures Ltd | Information & communication | 2023 |
| Massive Little World Limited | Arts & recreation | 2022 |
| Masstars Ventures Limited | Administrative & support | 2016 |
| Masstars Ventures Property Limited | Real estate | 2023 |
| Mast Group Holdings Limited | Real estate | 2018 |
| Master Building Care Ltd | Administrative & support | 2018 |
| Master Distribution Ltd | Transport & logistics | 2022 |
| Master Group Holdings Limited | Financial services | 2024 |
| Master Mindz Ltd | Information & communication | 2022 |
| Master Pass Limited | Education | 2019 |
| Mastercore Limited | Professional & scientific | 2011 |
| Masterkey Ampm Ltd | Real estate | 2024 |
| Masterline Ltd | Construction | 2025 |
| Masters Historic Racing Limited | Arts & recreation | 2005 |
| Masters Optical Services Ltd | Health & social care | 2024 |
| Masters Racing Club Limited | Arts & recreation | 2008 |
| Masterstroke Interiors Ltd | Construction | 2026 |
| Masthedecart.Re Ltd | Real estate | 2020 |
| Mastriv Ltd | Information & communication | 2026 |
| Masumin Motor Spares Limited | Wholesale & retail | 2011 |
| MAT Developments (Croydon) Holdings LLP | | 2019 |
| MAT Developments (Croydon) Limited | Construction | 2018 |
| MAT Developments (Winchester) Limited | Construction | 2015 |
| MAT Developments Services Limited | Construction | 2018 |
| MAT Resolutions Limited | Wholesale & retail | 2013 |
| Matador Holdings Ltd | Financial services | 2019 |
| Matarc Ltd | Information & communication | 2025 |
| Match Performance Solutions Limited | Administrative & support | 2015 |
| Matchday Vending Limited | Wholesale & retail | 2026 |
| Matchstick Property Ltd | Real estate | 2022 |
| Mate Natural Tea Ltd | Manufacturing | 2021 |
| Mateo&Marian Ltd | Construction | 2025 |
| Material in Mind Ltd | Professional & scientific | 2025 |
| Maternal Health Kiosk Ltd | Other services | 2024 |
| Maths Sensei Ltd | Education | 2025 |
| Matilda Lettings Limited | Real estate | 2024 |
| Matkis Farms and Agro Services Limited | Wholesale & retail | 2018 |
| Matmar Services Ltd | Construction | 2026 |
| Matop Ltd | Health & social care | 2024 |
| Matrai DRY Lining Ltd | Construction | 2014 |
| Matrix Folk Limited | Information & communication | 2022 |
| Matrix Global Networks Limited | Manufacturing | 2005 |
| Matrix Managed Marketplaces Ltd | Extraterritorial | 2019 |
| Matrix Precision Machining Ltd | Manufacturing | 2025 |
| Matrix SCM Group Ltd | Financial services | 2016 |
| Matrix SCM Limited | Administrative & support | 1988 |
| Matt Carney & Co Commercial Management Ltd | Professional & scientific | 2023 |
| Matt Coles Carpentry Ltd | Construction | 2019 |
| Matt Cooper Physiotherapy Limited | Other services | 2017 |
| Matt Crump Creative Ltd | Professional & scientific | 2026 |
| Matt DAW Lighting Design Limited | Arts & recreation | 2019 |
| Matt Fessey Lion Estates Ltd | Real estate | 2025 |
| Matt Parry Productions Ltd | Arts & recreation | 2023 |
| Mattdar Properties Ltd | Real estate | 2025 |
| Matthew Barrett Property Ltd | Real estate | 2021 |
| Matthew Brice Limited | Construction | 2015 |