| Merry Hill Development Management Limited | Households | 2024 |
| Merryfield Court (Stanmore) Residents Association Limited | Households | 1983 |
| Merryhill Stores Ltd | Wholesale & retail | 2017 |
| Mertons W Ltd | Construction | 2017 |
| Merza Limited | Professional & scientific | 2014 |
| Meshi Traders Limited | Wholesale & retail | 2024 |
| Mesilane Ltd | Construction | 2019 |
| MESKEN01 Limited | Hospitality | 2022 |
| Meta Accountancy Ltd | Professional & scientific | 2022 |
| Meta Executive Search Limited | Professional & scientific | 2014 |
| Meta Holdings Limited | Real estate | 2005 |
| Meta UK Maintenance Ltd | Construction | 2025 |
| Metal UK Limited | Professional & scientific | 2008 |
| Metali Drylining Ltd | Construction | 2019 |
| Metceeba Consultancy Ltd | Education | 2016 |
| Meteor Fire Protection Ltd | Construction | 2026 |
| Metric Building Consultancy Ltd | Professional & scientific | 2019 |
| Metric Index Limited | Information & communication | 2016 |
| Metric Sofas Ltd | Other services | 2026 |
| Metro College Ltd | Education | 2024 |
| Metro Local Retail Ltd | Wholesale & retail | 2011 |
| Metro Productions Ltd | Information & communication | 2014 |
| Metro Rent London Limited | Extraterritorial | 2021 |
| Metro Rent Ltd | Real estate | 2011 |
| Metropolitan & Suburban Limited | Construction | 2017 |
| Metropolitan Renewables Limited | Construction | 2026 |
| Mewa Distribution (UK) Limited | Wholesale & retail | 2020 |
| Mews Testing Ltd | Professional & scientific | 2018 |
| Mexeco Limited | Wholesale & retail | 2021 |
| Meyer Office Solutions Limited | Construction | 2014 |
| Mezza Luna (ST. Albans) Limited | Households | 2007 |
| Mfab Luxury Consulting Ltd. | Wholesale & retail | 2017 |
| MFAM14 Limited | Professional & scientific | 2025 |
| MFK Electric Ltd | Construction | 2024 |
| Mfortech Ltd | Information & communication | 2025 |
| MFP Management Ltd | Real estate | 2024 |
| MG Drainage Limited | Water & waste | 2000 |
| MG Gymstars Gymnastics Limited | Arts & recreation | 2018 |
| MG Miles Ltd | Transport & logistics | 2020 |
| MGB Advisors Ltd | Health & social care | 2022 |
| MGB London Limited | Construction | 2015 |
| Mgbotea Ltd | Construction | 2019 |
| MGC Bespoke UK Limited | Construction | 2016 |
| MGI Electrical Solutions Limited | Construction | 2018 |
| MGM Property Group Limited | Real estate | 2025 |
| MGM Surveying Services Ltd | Professional & scientific | 2012 |
| Mgri Network Consulting Ltd | Information & communication | 2023 |
| MGS Properties Essex Ltd | Real estate | 2020 |
| MGS Property Lettings Ltd | Real estate | 2018 |
| MGS Training Academy Limited | Education | 2026 |
| Mgsc Fit Ltd | Arts & recreation | 2025 |
| MH Crash Repairs Ltd | Wholesale & retail | 2022 |
| MH Realtors Ltd | Real estate | 2022 |
| Mhdc Management Ltd | Professional & scientific | 2025 |
| MHF Creative Ltd | Professional & scientific | 2013 |
| MHQ Consultants Limited | Information & communication | 2023 |
| MHR Consulting Solutions Ltd | Administrative & support | 2025 |
| Mhts Assets Ltd | Real estate | 2020 |
| MI Pro Services Limited | Construction | 2014 |
| MI Project Services Ltd | Construction | 2018 |
| MIA J Services Limited | Professional & scientific | 2019 |
| Miajames Ltd | Professional & scientific | 2019 |
| MIC Systems Limited | Information & communication | 2014 |
| Michael Connor Ltd | Professional & scientific | 2011 |
| Michael Ford Management Limited | Administrative & support | 2011 |
| Michael H Building Services Ltd | Construction | 2005 |
| Michael Zylka Ltd | Professional & scientific | 2024 |
| Michlin Limited | Construction | 2007 |
| Michmike Ltd | Health & social care | 2015 |
| Micro 7 Ltd | Health & social care | 2014 |
| Micro Construction Ltd | Construction | 2018 |
| Microgyro Limited | Professional & scientific | 2012 |
| Micronet Infotech Limited | Information & communication | 2004 |
| MID 2 Ltd | Health & social care | 2024 |
| MID Ocean Brands UK, Ltd | Wholesale & retail | 1998 |
| Midan Tech Consulting Ltd | Financial services | 2018 |
| Middle Earth Property Limited | Real estate | 2021 |
| Middle East Luxury Cars Limited | Real estate | 2018 |
| Middle Eight Music Limited | Information & communication | 1975 |
| Middlesex Construction Solutions Limited | Construction | 2013 |
| Midland Management Company ST Albans Limited | Households | 1998 |
| Midland Railway (Freehold) Company Ltd | Households | 2019 |
| Midlavic & Co Solicitors LLP | | 2025 |
| Midnightsparx Ltd | Construction | 2021 |
| Midsho Winery Limited | Manufacturing | 2021 |
| Mihyl Limited | Information & communication | 2013 |
| Mikafi Ltd | Professional & scientific | 2005 |
| Mikasa Concept Ltd | Construction | 2020 |
| Mikasa Homes Ltd | Real estate | 2023 |
| Mikelab Consultants Ltd | Professional & scientific | 2010 |
| MIL Logistic Ltd | Other services | 2018 |
| Milan Tanna Limited | Real estate | 2026 |
| Milbourne Court Residents Association (Watford) Limited | Other services | 1961 |
| Milcars (Radlett) Limited | Wholesale & retail | 1978 |
| Milcars Limited | Professional & scientific | 2003 |
| Mileadsys Ltd | Information & communication | 2014 |
| Milerun Limited | Other services | 1991 |
| Mill Hill Rugby Football Club Limited | Arts & recreation | 2015 |
| Mill Hill Village Sports Club Limited | Arts & recreation | 2012 |
| Millennium Care Holdings Limited | Financial services | 2024 |