| Marland Sykes Limited | Administrative & support | 2009 |
| Marlow Nutrition Co. Ltd | Health & social care | 2025 |
| Marlow Properties Ltd | Real estate | 2020 |
| Marlow Racquet Club Limited | Arts & recreation | 2025 |
| Marmor Engineering Ltd | Professional & scientific | 2015 |
| Marnee Limited | Professional & scientific | 1984 |
| Marples Ridgway Limited | Professional & scientific | 1911 |
| Marrbel Limited | Health & social care | 2015 |
| Martin O'Neill Consultants Ltd | Professional & scientific | 2017 |
| Marvel Mechanical Services Ltd | Construction | 2021 |
| Maselec Limited | Extraterritorial | 2015 |
| Massaoke Limited | Arts & recreation | 2006 |
| Mast Electrical Ltd | Construction | 2020 |
| Mastec Holdings Limited | Professional & scientific | 2003 |
| Master Consulting Services Ltd | Professional & scientific | 2021 |
| Masters of the Sky Ltd | Professional & scientific | 2023 |
| Matraves Services Ltd | Information & communication | 2023 |
| Matrix Financial Holdings Ltd | Financial services | 2018 |
| Matrix Hair Design Limited | Other services | 2009 |
| Matt Pilgrim Consulting Ltd | Information & communication | 2022 |
| Matt ROE Tattoo Limited | Other services | 2016 |
| Mattes and Miniatures Visual Effects Limited | Information & communication | 2006 |
| Mattfawcett Ltd | Real estate | 2022 |
| Mavanna Consultancy Limited | Professional & scientific | 2022 |
| Mavelstone Investments Limited | Financial services | 2014 |
| MAX Diego Limited | Wholesale & retail | 2024 |
| Maxadie Limited | Information & communication | 2012 |
| Maxen Consulting Limited | Professional & scientific | 2018 |
| Maxpertise Ltd | Wholesale & retail | 2008 |
| MAY School Services Ltd | Administrative & support | 2024 |
| Mayflower Wedding Ltd | Other services | 2022 |
| Maytone Limited | Wholesale & retail | 1971 |
| MB Electrical Consultancy Limited | Professional & scientific | 2022 |
| MC 449 Limited | Professional & scientific | 2008 |
| MC Ukproperty Ltd | Information & communication | 2019 |
| Mccandless Ltd | Professional & scientific | 2022 |
| Mccarthy Roofing Ltd | Construction | 2018 |
| Mccleary Home Improvements Ltd | Construction | 2026 |
| Mcevoy & Rowley HCO Limited | Professional & scientific | 1999 |
| Mcevoy and Rowley Services Limited | Wholesale & retail | 1982 |
| Mcewen Properties Ltd | Real estate | 2019 |
| Mcfarlane Telfer (Europe) Limited | Financial services | 2021 |
| Mcfarlane Telfer (GCC) Limited | Financial services | 2015 |
| Mcfarlane Telfer (KSA) Limited | Financial services | 2018 |
| Mcfarlane Telfer (Kuwait) Limited | Financial services | 2019 |
| Mcfarlane Telfer (Oman) Limited | Financial services | 2019 |
| Mcfarlane Telfer (Qatar) Limited | Financial services | 2015 |
| Mcfarlane Telfer (UAE) Limited | Financial services | 2014 |
| Mcfarlane Telfer Limited | Manufacturing | 1998 |
| Mcft Apac Ltd | Financial services | 2022 |
| Mcft Bowran Limited | Financial services | 2021 |
| Mcft Holdings Limited | Financial services | 1991 |
| Mcft Limited | Extraterritorial | 2018 |
| Mcft Philippines 1 Ltd | Financial services | 2022 |
| Mcft Philippines 2 Ltd | Financial services | 2022 |
| Mcgray Limited | Financial services | 2022 |
| Mckay Accountancy Services Ltd | Professional & scientific | 2024 |
| Mckay Luck Limited | Real estate | 2016 |
| Mckenzie & Watson Limited | Real estate | 2026 |
| Mcknight Estate Planning Ltd | Financial services | 2026 |
| Mcknight Wealth Management Ltd | Financial services | 2026 |
| Mclaren Euro Partners Limited | Professional & scientific | 2015 |
| MCM Facade Consulting Ltd | Professional & scientific | 2024 |
| Mcmx Marketing Ltd | Wholesale & retail | 2024 |
| Mcneil Jordan Consultancy Limited | Professional & scientific | 2024 |
| MCS Rental Software Limited | Information & communication | 2000 |
| MCS Solutions Holdings Ltd | Financial services | 2026 |
| MCS Solutions Limited | Information & communication | 1999 |
| Mcs-Uk Limited | Extraterritorial | 1998 |
| MD Motors Limited | Wholesale & retail | 2015 |
| MD Shah Ltd | Wholesale & retail | 2016 |
| MDF Auto Services Ltd | Wholesale & retail | 2025 |
| Mdktec Ltd | Information & communication | 2016 |
| ME Time Treatments Limited | Health & social care | 2014 |
| Mead Consulting Services Limited | Professional & scientific | 2023 |
| Mead Haze (Maidenhead) Residents Management Ltd | Households | 1973 |
| Meadow Cottage RES. Assoc. (Cookham) Limited | Real estate | 1996 |
| Meadow House Partners Limited | Financial services | 2024 |
| Meadoway Estates Limited | Real estate | 2023 |
| Meadowbrook Homes Limited | Construction | 1999 |
| Meadowbrook Management Company Limited | Households | 2003 |
| Meads Health Ltd | Health & social care | 2020 |
| Mecca Bingo Limited | Arts & recreation | 1984 |
| Medas Limited | Health & social care | 2014 |
| Medeo Ltd | Health & social care | 2023 |
| Mediapaedia Broadcast Limited | Administrative & support | 2015 |
| Medical and Regulatory Consulting Limited | Professional & scientific | 2015 |
| Medical Monitoring Consulting Limited | Professional & scientific | 2019 |
| Medicare Health Tourism Limited | Health & social care | 2015 |
| Medici Brothers Ltd | Professional & scientific | 2025 |
| Medusa Tanning Co Ltd | Other services | 2026 |
| Meeajaun Limited | Information & communication | 2005 |
| Meekvil Developments Limited | | 1957 |
| Mehak Textile Ltd | Wholesale & retail | 2025 |
| MEL Group Holdings Ltd | Professional & scientific | 2026 |
| MEL Legal Services Limited | Professional & scientific | 2021 |
| Mel's Bar Hospitality Limited | Hospitality | 2025 |
| Melia Publishing Services Limited | Information & communication | 1992 |
| Meliska Limited | Wholesale & retail | 2024 |
| Mellion Consulting Limited | Information & communication | 2020 |