| Marketing and Service Holdings Ltd | Financial services | 2022 |
| Marketing Sales Management Ltd | Professional & scientific | 2023 |
| Marley & Scrooge Limited | Extraterritorial | 2009 |
| Marmalade Skies Limited | Professional & scientific | 2015 |
| Marnozsales Limited | Professional & scientific | 2026 |
| Martell Holdings Limited | Real estate | 2021 |
| Martin and Mortimer Limited | Professional & scientific | 2014 |
| Marvell Green (Meldreth) Management Company Limited | Households | 2021 |
| Master Solutions Consultancy Ltd | Professional & scientific | 2024 |
| Masterplan Properties Limited | Administrative & support | 2002 |
| Matt Greenaway ATM Consulting Ltd | Transport & logistics | 2018 |
| Matt Langley Commercials Ltd | Wholesale & retail | 2026 |
| Maurer Instruments Limited | Manufacturing | 2002 |
| Maurer Technologies Limited | Professional & scientific | 2011 |
| Mavi Heating Ltd | Construction | 2025 |
| Maxi Grill Ltd | Hospitality | 2026 |
| Maximum Precision Ltd | Manufacturing | 2010 |
| Maxwell Melbourne Limited | Wholesale & retail | 2003 |
| MBA Property Consultants Limited | Professional & scientific | 1998 |
| MBM Resource Trading International Limited | Wholesale & retail | 1999 |
| MCB Legal Limited | Professional & scientific | 2023 |
| Mccormick Controls Ltd | Construction | 2022 |
| MCP Maintenance Limited | Manufacturing | 2025 |
| MCV BUS and Coach Limited | Manufacturing | 2002 |
| MDB Solutions (ELY) Limited | Construction | 2021 |
| Meadelsh Groups Limited | Hospitality | 2025 |
| Meadowcroft Air Conditioning & Refrigeration Limited | Energy | 2021 |
| Media Sauce UK Limited | Information & communication | 2006 |
| Mediascape Productions Limited | Administrative & support | 2007 |
| Medical Entomology Centre Limited | Professional & scientific | 1998 |
| Medicines Development for Global Health Limited | Health & social care | 2015 |
| MEG (Fordham) Limited | Arts & recreation | 2014 |
| Meggscreates Ltd | Information & communication | 2025 |
| Meltingshop Group Limited | Wholesale & retail | 2015 |
| Melvyn Connell Ltd | Information & communication | 2018 |
| Ment Surveying Services Limited | Professional & scientific | 2016 |
| Mental Health and Wellbeing Training Ltd | Health & social care | 2018 |
| Merci SWT Ltd | Information & communication | 2021 |
| Mercurial CAD Services Ltd | Professional & scientific | 2010 |
| Mereside Medical Services Limited | Health & social care | 2006 |
| Meriva Direct Ltd | Construction | 2023 |
| Meriva Estates Ltd | Real estate | 2024 |
| Meriva Plus Limited | Construction | 2016 |
| Metamorphosis the Gallery Ltd | Wholesale & retail | 2022 |
| Metcalfe Tennis Club Limited | Arts & recreation | 2000 |
| Metering & Technology Limited | Administrative & support | 2006 |
| Methanum Buntingford Limited | Energy | 2018 |
| Methanum Limited | Financial services | 2015 |
| Methanum Property Limited | Real estate | 2017 |
| Methanum Thaxted Limited | Administrative & support | 2017 |
| MGA and Associates (Property) Limited | Administrative & support | 2008 |
| MGI Distribution Limited | Wholesale & retail | 2024 |
| Mi.R.A. Car Parts Ltd | Wholesale & retail | 2021 |
| Micah Community Development and Education Services Limited | Education | 2024 |
| Michael Herd Consulting Ltd | Professional & scientific | 2019 |
| Michael Michaels Art Gallery Ltd | Wholesale & retail | 2025 |
| Micron Alpha Limited | Professional & scientific | 2001 |
| Micron Research Limited | Professional & scientific | 1996 |
| Middle J Carpentry Limited | Manufacturing | 2022 |
| Midmodretro Ltd | Wholesale & retail | 2020 |
| Miky Homecare Ltd | Health & social care | 2021 |
| Mil-Ken Travel Limited | Transport & logistics | 1983 |
| Miledaise Limited | Real estate | 1988 |
| Military Auto Sales Ltd | Wholesale & retail | 2017 |
| Mill Field (Sutton) Residents Management Limited | Households | 2026 |
| Millard Consultants Ltd | Real estate | 2020 |
| Miller Brothers Coaches Limited | Transport & logistics | 2013 |
| Millers Environmental & Sons Limited | Transport & logistics | 2012 |
| Millers Haulage Limited | Transport & logistics | 2018 |
| Millers Partners Limited | Transport & logistics | 2006 |
| Millers Property Management Limited | Construction | 2018 |
| Millfield Lane Designs Ltd | Professional & scientific | 2023 |
| Millrose Telecom Limited | Information & communication | 2014 |
| Mimas Limited | Professional & scientific | 1998 |
| Mind Fuse Media Ltd | Professional & scientific | 2025 |
| Miniature Kingdoms Ltd | Wholesale & retail | 2024 |
| Minu Estates Ltd | Real estate | 2022 |
| Miroslaw Staniewski Ltd | Transport & logistics | 2021 |
| Mister Boiler Limited | Other services | 2011 |
| Mitchell & Mayle Ltd | Water & waste | 2013 |
| Mitreaconstruct Ltd | Construction | 2020 |
| Mitzy Limited | Administrative & support | 2021 |
| Mixco (Cambridge) Ltd | Manufacturing | 2015 |
| Mixco Services Limited | Wholesale & retail | 2017 |
| MJ Engineering Consultancy Ltd | Professional & scientific | 2025 |
| MK Demolition & Reclaim Ltd | Construction | 2025 |
| MK Principal Ltd | Professional & scientific | 2025 |
| MLH Transport Limited | Transport & logistics | 2008 |
| Mngrounwork Ltd | Construction | 2015 |
| MO Guled Ltd | Health & social care | 2015 |
| Mobile RF Systems Limited | Information & communication | 2008 |
| Mobility Renewed Ltd | Wholesale & retail | 2025 |
| Mobilitycare Bathrooms Ltd | Construction | 2025 |
| Modicum Planning Ltd | Professional & scientific | 2017 |
| Mole Architects Limited | Professional & scientific | 2002 |
| Monarchs Place (Rickling Green) Limited | Households | 2020 |
| Monisvick Limited | Administrative & support | 2020 |
| Monkfield Nutrition Limited | Wholesale & retail | 2005 |
| Monroe Motorsport Ltd | Wholesale & retail | 2026 |
| Montague Engineering Limited | Manufacturing | 2014 |