| M & S Carpentry Contractors Limited | Construction | 2002 |
| M & SS (UK) Ltd | Wholesale & retail | 2017 |
| M a Birch Civil Engineering Ltd | Construction | 2022 |
| M a Birch Holdings Limited | Financial services | 2026 |
| M a Birch Structured Cabling Limited | Other services | 2013 |
| M Allum Trading Ltd | Hospitality | 2026 |
| M B a Inflatables Ltd | Arts & recreation | 2014 |
| M B Contracts Limited | Construction | 2008 |
| M B E Marketing Limited | Professional & scientific | 2011 |
| M B Grainger Services Limited | Wholesale & retail | 2017 |
| M B Hairdressing Limited | Other services | 2024 |
| M C Jarvis Ltd | Agriculture & fishing | 2012 |
| M C Simons Fire Safety Services Ltd | Professional & scientific | 2022 |
| M D Automotive Limited | Other services | 2013 |
| M Designs and Engineering Ltd | Professional & scientific | 2021 |
| M Fulcher Contracts Limited | Construction | 2026 |
| M G Electric (Colchester) Limited | Manufacturing | 1966 |
| M Hopkins Plumbing & Heating Ltd | Construction | 2014 |
| M J Enoch Tree Surgery Limited | Agriculture & fishing | 2011 |
| M J Glazing Limited | Construction | 2003 |
| M J S Interiors Ltd | Construction | 2015 |
| M J T Partners Ltd | Professional & scientific | 2015 |
| M Loader Financial Services Ltd | Financial services | 2022 |
| M M VEG Colchester Ltd. | Wholesale & retail | 2026 |
| M Marriott Construction Ltd | Construction | 2021 |
| M Motors Limited | Wholesale & retail | 2021 |
| M P Carpentry Contracts Limited | Construction | 2023 |
| M P Halls Limited | Real estate | 2006 |
| M P London Ltd | Construction | 2013 |
| M R Currell Limited | Construction | 2002 |
| M Raymond Electrical Ltd | Construction | 2010 |
| M S Lawrence and Associates Ltd | Other services | 2019 |
| M Scott Properties Limited | Construction | 2008 |
| M Scott Property Group Limited | Professional & scientific | 2009 |
| M Star Barber Ltd | Other services | 2022 |
| M Steels Limited | Administrative & support | 2017 |
| M V Autos Limited | Wholesale & retail | 2013 |
| M W Aiken Consulting Ltd | Professional & scientific | 2025 |
| M Whitehead Tree Care Ltd | Administrative & support | 2025 |
| M&B Cottages Limited | Real estate | 2025 |
| M&B Property Investment Ltd | Real estate | 2017 |
| M&C Diesel Services Limited | Wholesale & retail | 2002 |
| M&E Systems Ltd | Information & communication | 2025 |
| M&G Lion Ltd | Hospitality | 2019 |
| M&Jht Ltd | Professional & scientific | 2026 |
| M&L Insulation Contractors Limited | Construction | 2025 |
| M&M Painting Ltd | Construction | 2013 |
| M&R GUT Service Ltd | Health & social care | 2019 |
| M&S Parratt Livestock Limited | Agriculture & fishing | 2003 |
| M&Sm (UK) Ltd | Wholesale & retail | 2013 |
| M&Yz Limited | Professional & scientific | 2015 |
| M&Z CAB Ltd | Transport & logistics | 2024 |
| M-Brace U Ltd | Arts & recreation | 2022 |
| M-Peak Technology Limited | Information & communication | 2007 |
| M. Barnett Carpentry Ltd | Construction | 2023 |
| M. Canfield and SON Ltd | Construction | 2021 |
| M. J. Green Treework (Wivenhoe) Ltd | Agriculture & fishing | 2005 |
| M. J. S. Systems Limited | Administrative & support | 1999 |
| M. Scott Properties LLP | | 2006 |
| M.A Scaffolding Limited | Construction | 2017 |
| M.A.Dbathroomco Ltd | Construction | 2024 |
| M.D.T. Services Ltd | Other services | 1996 |
| M.F.D. Ductwork Installations Limited | Construction | 1989 |
| M.G.T. Services Ltd | Construction | 2002 |
| M.K. Engineering (Colchester) Limited | Manufacturing | 2002 |
| M.O.F Property Limited | Real estate | 2021 |
| M.R.S.Hubinns Limited | Hospitality | 2026 |
| M.S.G Carpenters Ltd | Construction | 2009 |
| M.Y Reliable Properties Limited | Real estate | 2025 |
| M3S Consulting Ltd | Information & communication | 2020 |
| MA Business Support Ltd | Professional & scientific | 2019 |
| Ma&Mr Partners Limited | Real estate | 2025 |
| MAB Building Solutions Ltd | Construction | 2009 |
| MAB Planning Solutions Ltd | Construction | 2015 |
| MAB Roofing Limited | Construction | 2006 |
| MAB Solar Limited | Construction | 2022 |
| Mabon Design and Print Limited | Professional & scientific | 2021 |
| Mabouya Ltd | Hospitality | 2023 |
| MAC Ventures Group Limited | Hospitality | 2019 |
| Macaii Limited | Professional & scientific | 2019 |
| Macdon Global Ltd | Wholesale & retail | 2025 |
| Macdowell Medical Services Ltd | Health & social care | 2024 |
| Mace Housekeeping Ltd | Administrative & support | 2021 |
| Maceroad Ltd | Real estate | 2020 |
| Machy's Bakery Limited | Hospitality | 2025 |
| Macintyre Simmonds Limited | Extraterritorial | 2010 |
| Mack Consultancy Ltd | Extraterritorial | 2024 |
| Mack Property Holdings Ltd | Real estate | 2026 |
| Mackay Tech Consulting Ltd | Administrative & support | 2019 |
| Mackazines Ltd | Wholesale & retail | 2026 |
| Mackey’S Mind Ltd | Wholesale & retail | 2020 |
| Mackim Ltd | Professional & scientific | 2011 |
| Macnamara Civils Limited | Construction | 2020 |
| Macritchie Made Limited | Manufacturing | 2024 |
| Macro Bros Ltd | Extraterritorial | 2023 |
| Macula Ltd | Professional & scientific | 2023 |
| MAD About Sport Limited | Wholesale & retail | 2004 |
| MAD Cats Cattery Ltd | Agriculture & fishing | 2017 |
| MAD Little Marketer Ltd | Professional & scientific | 2023 |
| Maddisons (UK) Limited | Wholesale & retail | 2011 |