| Melmott Court Freehold Company Limited | Households | 2015 |
| Melrosa Holdings Limited | Real estate | 2026 |
| Memories in Verse Ltd. | Wholesale & retail | 1993 |
| Mentionsocials Ltd | Professional & scientific | 2022 |
| Mentor Group Limited | Professional & scientific | 1992 |
| Mentor Holdings Limited | Financial services | 2023 |
| Merchandnice Ltd | Wholesale & retail | 2020 |
| Meridian Botley Limited | Hospitality | 2001 |
| Meridian Hall Limited | Real estate | 1995 |
| Meridian Leisure Management Limited | Administrative & support | 1994 |
| Meridian Redditch Limited | Extraterritorial | 2003 |
| Merlin Blakeney Direct Limited | Administrative & support | 2002 |
| Merlin Direct Supplies Limited | Wholesale & retail | 2021 |
| Mero Consultancy and Management Ltd | Real estate | 2022 |
| Merpas (UK) Limited | Financial services | 2017 |
| Merrett Capital Limited | Financial services | 2013 |
| Merrittocracy Limited | Information & communication | 2009 |
| Merryfield Trading Ltd | Wholesale & retail | 2025 |
| Merseyside Energy Recovery Holdings Ltd | Professional & scientific | 2012 |
| Merseyside Energy Recovery Ltd | Water & waste | 2012 |
| Messum Yachts Limited | Other services | 2011 |
| Metafence Limited | Extraterritorial | 2016 |
| Metainy Air Conditioning Limited | Construction | 2000 |
| Metainy Service Limited | Construction | 2012 |
| Metalcor International Ltd | Wholesale & retail | 2007 |
| Metalium Curtain Wall Company UK Limited | Extraterritorial | 2021 |
| Metavasis Limited | Information & communication | 2014 |
| Meters Direct Ltd | Professional & scientific | 2025 |
| MEW Property Limited | Real estate | 2006 |
| Meyshia Limited | Information & communication | 2006 |
| Mezaro Energy Limited | Professional & scientific | 2000 |
| Mezaro Limited | Professional & scientific | 2003 |
| MG Bridge Investments Ltd | Professional & scientific | 2014 |
| MGA Professional Services Limited | Professional & scientific | 2007 |
| MGD Global Limited | Wholesale & retail | 2022 |
| Mghj Ltd | Information & communication | 2022 |
| Mgmt Consultancy Services Limited | Professional & scientific | 2012 |
| MGS Aesthetics Limited | Other services | 2022 |
| MGS Design Limited | Other services | 2021 |
| MH Haulage Limited | Transport & logistics | 2016 |
| Mhc(Gb) Ltd | Administrative & support | 2008 |
| Mhkz Ltd | Hospitality | 2016 |
| Mhofu Ltd. | Health & social care | 2012 |
| MI Casa SU Casa Services Limited | Real estate | 2019 |
| Michael Allen (Maintenance) Limited | Manufacturing | 2003 |
| Michael IT Consultancy Services Ltd | Information & communication | 2021 |
| Michael Parkinson Enterprises Limited | Information & communication | 1971 |
| Michele Giocchino Michael Azzaretto Receiver Limited Partnership | | 2014 |
| Mick George Concrete Limited | Manufacturing | 2012 |
| Mick George Contracting Limited | Administrative & support | 2010 |
| Mick George Demolition Limited | Professional & scientific | 2012 |
| Mick George Earthworks Limited | Water & waste | 2015 |
| Mick George EBT Trustee Limited | Professional & scientific | 2021 |
| Mick George Environmental Limited | Water & waste | 2016 |
| Mick George Limited | Mining & quarrying | 1989 |
| Mick George Mepal Limited | Professional & scientific | 1997 |
| Mick George Recycling Limited | Water & waste | 2017 |
| Micropenny Limited | Information & communication | 2009 |
| Microshare Ltd | Information & communication | 2013 |
| Microsuction Solutions Ltd | Health & social care | 2025 |
| Microtech Global Ltd | Administrative & support | 2012 |
| Microwave Consultants Ltd | Professional & scientific | 2011 |
| Middlegate Management Limited | Professional & scientific | 1999 |
| Middlesex and Surrey Commercials Limited | Wholesale & retail | 2011 |
| Midland Land Reclamation Limited | Water & waste | 1983 |
| Midland Quarry Products Limited | Mining & quarrying | 1996 |
| Midlife Drip Ltd | Professional & scientific | 2026 |
| Mighty Concrete Ltd | Manufacturing | 2024 |
| Mighty Iron Bear Ltd | Wholesale & retail | 2025 |
| Mighty Willow Limited | Arts & recreation | 2014 |
| Mightycrete Ltd | Manufacturing | 2019 |
| Mike Perkins Electrical Ltd | Construction | 2015 |
| Mike Ward Mortgage Services Ltd | Financial services | 2021 |
| Mike's Complete Construct Limited | Construction | 2022 |
| Milano Fresh UK Ltd | Wholesale & retail | 2021 |
| Mildreds Holdings Limited | Financial services | 2018 |
| Miles Ahead Services Ltd | Manufacturing | 2020 |
| Mileshield Commercial Funding Limited | Financial services | 1986 |
| Mileshield Limited | Administrative & support | 1985 |
| Milestone Advisory Limited | Professional & scientific | 2022 |
| Millah Homes Ltd | Real estate | 2022 |
| Millah Properties Ltd | Real estate | 2020 |
| Millenium Star Limited | Administrative & support | 2020 |
| Miller Lane (Littlewick) Management Limited | Households | 2013 |
| Millertech Limited | Manufacturing | 2014 |
| Million Step Foundation CIC | Education | 2026 |
| Milton Hall(Southend)Brick Company Limited(The) | Professional & scientific | 1878 |
| Milton Hayes Limited | Professional & scientific | 2013 |
| Milvus Residents Limited | Real estate | 2025 |
| Mimir Education and Training Consultancy Ltd | Professional & scientific | 2025 |
| Mind Matrix UK Limited | Professional & scientific | 2018 |
| Mindbrick Ltd | Construction | 2021 |
| Mindtactics Consultancy Ltd | Information & communication | 2023 |
| Minerva Inc Ltd | Professional & scientific | 2024 |
| Ming Way Limited | Information & communication | 2018 |
| Minim Games Ltd | Extraterritorial | 2021 |
| Miniracing.Com Limited | Administrative & support | 2018 |
| Minster Quarries Limited | Financial services | 2004 |
| Mint Maintenance Services Ltd | Administrative & support | 2020 |
| Mintaka International (OIL & GAS) Limited | Professional & scientific | 2003 |