| Meadowbank Homes LLP | | 2021 |
| Meadowson Limited | Construction | 1977 |
| Meadowson Properties Limited | Real estate | 2005 |
| MEC Medical Limited | Manufacturing | 2004 |
| Mechanical Structures Limited | Professional & scientific | 2020 |
| Mechline Consulting Limited | Professional & scientific | 2025 |
| Media House Hertford RTM Company Limited | Households | 2026 |
| Media Hub Ltd | Information & communication | 2016 |
| Media Solutions Global Ltd | Professional & scientific | 2025 |
| Mediacoms Limited | Other services | 2001 |
| Medicbear Ltd | Information & communication | 2025 |
| Mediscan Healthcare Ltd | Health & social care | 2007 |
| Mediscan Holdings Limited | Financial services | 2018 |
| Meerkat Marketing Limited | Professional & scientific | 2007 |
| Meetandgreet VIP Services Limited | Other services | 2011 |
| Melissa Amos Academy Limited | Other services | 2024 |
| Melrose Place Freehold Limited | Households | 2012 |
| Melrose Place Management Company (NO.1) Limited | Households | 1991 |
| Melrose Place Management Company (NO.2) Limited | Households | 1991 |
| Melvis Ltd | Real estate | 2019 |
| Meredith Holmes & Mcnicholas Limited | Construction | 2003 |
| Meridian Prestige Ltd | Wholesale & retail | 2019 |
| Merlin Integrated Solutions Limited | Information & communication | 2013 |
| Merta Limited | Hospitality | 2020 |
| Merton Rise (Hemel Hempstead) Management Limited | Households | 2022 |
| Mesa Latina DOS Limited | Professional & scientific | 2020 |
| Metallurgica Consult Ltd | Professional & scientific | 2019 |
| Mfd-Uk Limited | Wholesale & retail | 2022 |
| MGH Roofline Limited | Construction | 2022 |
| MGL Projects Limited | Construction | 2012 |
| MGM Design and Build Limited | Construction | 1997 |
| MGV Group Ltd | Construction | 2021 |
| MHM Media Ltd | Professional & scientific | 2015 |
| Mhss Trading Ltd | Professional & scientific | 2024 |
| MHW1 Limited | Professional & scientific | 2015 |
| Michael Ross Limited | Arts & recreation | 1937 |
| Michelle Fiedler Design Limited | Professional & scientific | 2014 |
| Michelle Louise Consulting Limited | Professional & scientific | 2022 |
| Mickys Courier & Transport Ltd | Transport & logistics | 2019 |
| Middlemarch Interiors Ltd | Other services | 2020 |
| Miilk Ltd | Wholesale & retail | 2018 |
| Mikael Limited | Information & communication | 2010 |
| Mike Gatting Limited | Arts & recreation | 1999 |
| Mikemo Limited | Administrative & support | 2021 |
| Milebay Auditing Limited | Other services | 2020 |
| Military History Group Limited | Information & communication | 2023 |
| Milksha (UK) Ltd | Hospitality | 2021 |
| Millbridge Management Company Limited | Real estate | 1996 |
| Millbrook Residents Limited | Real estate | 2000 |
| Millburn & Partners Limited | Administrative & support | 2013 |
| Millers Quarter Management Company Limited | Households | 2018 |
| Millstream Property Investments Ltd | Financial services | 2018 |
| Millstrom PUB Ltd | Hospitality | 2026 |
| Millwood Investments (Stanmore) Ltd | Professional & scientific | 2015 |
| Mina Andala Limited | Information & communication | 2016 |
| Mind Savvy Limited | Health & social care | 2023 |
| Minds EN Place Ltd | Health & social care | 2019 |
| Minerva Cloud Consulting Limited | Information & communication | 2017 |
| Ming Holdings Limited | Real estate | 2023 |
| Mingo Limited | Other services | 2016 |
| Minipartment Limited | Administrative & support | 2022 |
| Mint Nutrition Ltd | Health & social care | 2026 |
| Mirage (College Road Sandhurst) Management Company Limited | Households | 2006 |
| Mirage Line Entertainment Ltd | Information & communication | 2024 |
| Miranda Bolter Studio Limited | Professional & scientific | 2026 |
| Miss Disco Ltd | Arts & recreation | 2021 |
| Mission 4 Recruitment Limited | Administrative & support | 2013 |
| Mission Environmental Limited | Administrative & support | 2009 |
| Mitregrid Limited | Construction | 2025 |
| MIU Design Limited | Administrative & support | 2010 |
| MJG Transport Limited | Transport & logistics | 2014 |
| MJL Business Services Limited | Administrative & support | 2002 |
| MKB Services (Herts) Ltd | Professional & scientific | 2019 |
| MM Electrics Limited | Construction | 2024 |
| Mmere Dane UK (Mduk) Ltd | Wholesale & retail | 2024 |
| MN Consultancy PVT Ltd | Professional & scientific | 2016 |
| Mobel Limited | Hospitality | 2018 |
| Mobile Vehicle Mechanic Limited | Wholesale & retail | 2008 |
| Mode Garden Rooms Ltd | Construction | 2002 |
| Modern Baby Limited | Real estate | 2006 |
| Modern Worktops Limited | Construction | 2012 |
| Modern World Management Ltd | Professional & scientific | 2024 |
| Modifica Motor Ltd | Wholesale & retail | 2023 |
| Modus Property Services Limited | Construction | 2012 |
| Modus Publishing Group Limited | Information & communication | 2022 |
| Modus Umbrella Ltd | Administrative & support | 2020 |
| Molewood Park Residents Limited | Households | 1995 |
| Molines Wharf Management Company Limited | Households | 1998 |
| Monarch Design & Build Limited | Extraterritorial | 2001 |
| Money Savvy Limited | Information & communication | 2019 |
| Monkswood (Waltham Abbey) Management Limited | Households | 1993 |
| Monmouth Court (Northwood) Management Company Limited | Households | 2008 |
| Montu Consultance Limited | Professional & scientific | 2017 |
| MOO Pimlico Limited | Hospitality | 2013 |
| Moomoo Cowes Ltd | Construction | 2016 |
| Mooncow Limited | Wholesale & retail | 2016 |
| Moorings Management (Hertford) Limited | Households | 2005 |
| More Mechanical Services Limited | Construction | 2006 |
| Morello Apps Ltd | Information & communication | 2022 |
| Morizzo & Co Limited | Professional & scientific | 2018 |