| Melos Limited | Arts & recreation | 2010 |
| Melrose Tynan Limited | Arts & recreation | 2012 |
| Memmi Mortgages Ltd | Financial services | 2025 |
| Memphies Ltd | Wholesale & retail | 2022 |
| Mendoza Cafe Bar Limited | Hospitality | 2018 |
| Menghan & a UK Limited | Real estate | 2025 |
| Menrfa Ltd | Professional & scientific | 2015 |
| Mensah Consulting Limited | Financial services | 2007 |
| Mensahbrew Limited | Transport & logistics | 2017 |
| Menth Limited | Health & social care | 2025 |
| Mentina Therapies Limited | Health & social care | 2025 |
| Meraki Consultancy Limited | Professional & scientific | 2019 |
| Merbee Food Limited | Hospitality | 2017 |
| Mercury Information Systems Limited | Information & communication | 2013 |
| Meridian Verve Limited | Real estate | 2024 |
| Merlin Wealth Planning Limited | Financial services | 2018 |
| Merr MEP Ltd | Professional & scientific | 2026 |
| Merriclean Limited | Other services | 2012 |
| Merrrch Ltd | Manufacturing | 2017 |
| Merryweather Orchard Limited | Real estate | 2018 |
| Mesce Properties Limited | Real estate | 2019 |
| Meshl Limited | Information & communication | 2025 |
| Meshuga Consulting Limited | Information & communication | 2016 |
| Meskel Ltd | Wholesale & retail | 2025 |
| Metalkraft Ltd | Manufacturing | 2006 |
| Metamorphosis Consulting 2024 Ltd | Professional & scientific | 2024 |
| Metamorphosis Uysp CIC | Education | 2024 |
| Metamoulds Limited | Manufacturing | 1966 |
| Metataxis Limited | Information & communication | 2002 |
| Metcalfe Motorsport Ltd | Arts & recreation | 2023 |
| Metdesk Limited | Information & communication | 2012 |
| Meter Monitoring Services Ltd | Administrative & support | 2012 |
| Metis Insights Ltd | Information & communication | 2025 |
| Metre Squared Interiors Ltd | Agriculture & fishing | 2004 |
| Metro Food Delivery Ltd | Hospitality | 2020 |
| Metro Group(Gb) Ltd | Wholesale & retail | 2018 |
| Metro Voyage Private Limited | Administrative & support | 2025 |
| Metroserve Cleaning Ltd | Administrative & support | 2016 |
| Mewden Limited | Wholesale & retail | 2025 |
| Mews Homes Limited | Construction | 2018 |
| Mexiconti Ventures Ltd | Wholesale & retail | 2024 |
| Meyo Property Ltd | Real estate | 2022 |
| Mezmee Ltd | Information & communication | 2015 |
| Mezz One Ltd | Professional & scientific | 2020 |
| MF (Aylesbury) Limited | Wholesale & retail | 2019 |
| MF (Stevenage) Ltd | Wholesale & retail | 2019 |
| MF Equipment (UK) LLP | | 2004 |
| MF Equipment Limited | Administrative & support | 1988 |
| MFB Engineering Solutions Ltd | Professional & scientific | 2022 |
| MG Contracts and Safety Solutions Ltd | Construction | 2026 |
| MG Precious Property Ltd | Real estate | 2023 |
| MG Prime Ltd | Construction | 2015 |
| MG Techno Limited | Information & communication | 2013 |
| MGEN131. Ltd | Construction | 2024 |
| MGR Logistics Ltd | Transport & logistics | 2025 |
| MGR Plumbing & Heating Ltd | Construction | 2016 |
| MH Redhill Construction Limited | Financial services | 2019 |
| MHB and ZLB Ltd | Health & social care | 2025 |
| Mhbh IT Solutions Limited | Information & communication | 2022 |
| Mhka Properties Limited | Real estate | 2025 |
| MI Capital Research Limited | Financial services | 2013 |
| MI Care Clinic Ltd | Health & social care | 2026 |
| MIC 786 Ltd | Transport & logistics | 2018 |
| Micha Design Company Limited(The) | Manufacturing | 1986 |
| Micha Solar Limited | Extraterritorial | 2015 |
| Michael Day Aerial & Satellite Supplies Ltd | Wholesale & retail | 2010 |
| Michael Patterson Engineering & Restoration Limited | Manufacturing | 2015 |
| Mick Riley Building and Landscaping Services Limited | Construction | 2013 |
| Mick Surman Motorcycles Ltd | Wholesale & retail | 2014 |
| Micklefield Pre-School Ltd | Education | 2016 |
| Midas Training Solutions Limited | Information & communication | 1997 |
| Middle-Earth Enterprises Limited | Manufacturing | 2007 |
| Middleton Flooring Ltd | Wholesale & retail | 2022 |
| Midkimble Limited | Professional & scientific | 2015 |
| Mighty Consulting Limited | Financial services | 2025 |
| Mighty J African Wholesale Groceries Limited | Wholesale & retail | 2025 |
| Mighty J Limited | Wholesale & retail | 2021 |
| Mighty Oaks Limited | Professional & scientific | 2006 |
| Mikes Cars Ltd | Transport & logistics | 2023 |
| Milan Interiors Limited | Construction | 2022 |
| Milicargo Ltd | Transport & logistics | 2025 |
| Milkay Ltd | Health & social care | 2013 |
| Mill Court Management (Waddesdon) Limited | Households | 1988 |
| Mill Mead (Wendover) Management Limited | Other services | 1997 |
| Millbrook Land Planning & Development Limited | Real estate | 2002 |
| Millennium FX Limited | Administrative & support | 1999 |
| Miller Commercial Motors Limited | Wholesale & retail | 2017 |
| Miller Mcenroe Ltd | Manufacturing | 2020 |
| Milly Digital Ltd | Information & communication | 2023 |
| Milspares Limited | Wholesale & retail | 2003 |
| Milton Build Limited | Construction | 2017 |
| Milton Gardens Management Company Limited | Households | 1987 |
| Milton Keynes Reclamation Ltd | Construction | 2012 |
| Milvus Hill Limited | Financial services | 2017 |
| Mimi Trans Ltd | Transport & logistics | 2019 |
| Mims NN Ltd | Health & social care | 2014 |
| Mindblu Technologies Ltd | Information & communication | 2020 |
| Minds Alike Animal Training and Behaviour Limited | Agriculture & fishing | 2021 |
| Minerva Stoneworks Ltd | Manufacturing | 2023 |
| Minibusing Ltd | Information & communication | 2018 |