| MAS Contracting Limited | Construction | 2018 |
| MAS Whizz Digitals Limited | Information & communication | 2025 |
| Masala Mahal Ltd | Hospitality | 2023 |
| Mason Archer Limited | Administrative & support | 2009 |
| Master GAS Limited | Construction | 2017 |
| Master Makeup With Chloe Sims Ltd | Other services | 2021 |
| Master Plastering Ltd | Administrative & support | 2016 |
| Master Roofing London Ltd | Construction | 2021 |
| Matchcast Limited | Administrative & support | 2015 |
| Matchpoint Recruitment (UK) Limited | Administrative & support | 2010 |
| Matrix Contracting Ltd | Construction | 2023 |
| Matrixx Estates Limited | Real estate | 2023 |
| Matthews Investments Holdings Ltd | Construction | 2020 |
| Mauj Events Limited | Administrative & support | 2025 |
| Mauj Ltd | Wholesale & retail | 2025 |
| Maurice House Limited | Financial services | 2018 |
| MAX Group Recruitment Ltd. | Construction | 2025 |
| MAX London Group Limited | Construction | 2026 |
| Maxillicious Limited | Professional & scientific | 2025 |
| Maya & Ayla Limited | Wholesale & retail | 2025 |
| Maybro Limited | Arts & recreation | 2025 |
| Mayflower Court Residents Association Limited | Households | 1993 |
| Mayflower Financial Solutions Limited | Financial services | 2025 |
| Mayne Lavelle Limited | Professional & scientific | 2010 |
| Mayne Services LLP | | 2010 |
| Maypole Gardens (Hainault) Residents Company Limited | Households | 1986 |
| MB Civil Engineers Ltd | Construction | 2020 |
| MB Post Production Limited | Information & communication | 2025 |
| MB Roofing Services (London & Herts) Limited | Construction | 2021 |
| MBR Fruit & VEG Limited | Wholesale & retail | 2023 |
| Mcdougall Limited | Financial services | 1999 |
| MCF Services Ltd | Construction | 2017 |
| MCG Software Solutions Ltd | Information & communication | 2019 |
| Mckay Properties Ltd | Real estate | 2022 |
| Mckenzie Brothers (East Anglia) Limited | | 1986 |
| Mckinnon's Interiors Ltd | Construction | 2019 |
| MCL Electrical Services Limited | Construction | 2011 |
| MCS Lifts Limited | Administrative & support | 2010 |
| Mctrout Properties Limited | Administrative & support | 2023 |
| MD Desserts Ltd | Hospitality | 2025 |
| MD Furniture & Decoration Ltd | Manufacturing | 2024 |
| MD Grocery Ltd | Wholesale & retail | 2019 |
| MD Smart Services Limited | Professional & scientific | 2018 |
| MDA Consulting Engineers Ltd | Professional & scientific | 2019 |
| MDC Property Investment Limited | Real estate | 2021 |
| Mdht Ltd | Health & social care | 2023 |
| MDN Fixing Limited | Construction | 2015 |
| MDS Construction Services Limited | Construction | 2017 |
| Mdt-Tex UK Ltd | Manufacturing | 2020 |
| ME and ROO Ltd | Wholesale & retail | 2023 |
| Mead House Management Limited | Households | 1999 |
| Mead House Real Estate Ltd | Real estate | 2024 |
| Meadow Rose Limited | Professional & scientific | 2017 |
| Meadows Electrics (Wanstead) Limited | Other services | 2006 |
| Meadway Private Hire Limited | Transport & logistics | 1969 |
| Meadway Radio Cars Limited | Professional & scientific | 2006 |
| Meat Plant Ltd | Hospitality | 2023 |
| Mecasonic (UK) Limited | Administrative & support | 2005 |
| Media 10 Bidco Limited | Information & communication | 2021 |
| Media 10 Construction Limited | Information & communication | 2021 |
| Media 10 Design Holdings Limited | Financial services | 2021 |
| Media 10 Holdings Limited | Professional & scientific | 2009 |
| Media 10 Home Interest Holdings Limited | Financial services | 2021 |
| Media 10 Limited | Information & communication | 2002 |
| Media 10 Ventures Limited | Information & communication | 2001 |
| Mediation Consulting Ltd | Professional & scientific | 2018 |
| Medical Skin Ltd | Health & social care | 2026 |
| Medical Waste Ltd | Water & waste | 2025 |
| Medina Gallions Limited | Real estate | 2024 |
| Medlab Peptides Ltd | Wholesale & retail | 2026 |
| Mednurse Personnel Limited | Administrative & support | 2022 |
| Medprep Ltd | Education | 2019 |
| Mefb Limited | Hospitality | 2025 |
| Mega Co Group Ltd | Construction | 2022 |
| Megabooth Ltd | Arts & recreation | 2010 |
| Megan Holly Ltd | Wholesale & retail | 2022 |
| Meissa Property Management Ltd | Real estate | 2017 |
| MEL Salon Loughton Ltd | Other services | 2026 |
| Melly Cares Ltd | Health & social care | 2021 |
| Melly Loves Limited | Health & social care | 2025 |
| Melrose Property Services Ltd | Real estate | 2018 |
| Memon Imports Limited | Wholesale & retail | 2006 |
| Men's ZEN Ltd | Wholesale & retail | 2021 |
| Mensah Property Development Ltd | Real estate | 2026 |
| Mentor Coach Partners Ltd | Information & communication | 2013 |
| Meon Maintenance Ltd | Administrative & support | 2021 |
| Meralon International Limited | Professional & scientific | 2006 |
| Merchants Place Management Company Limited | Households | 2003 |
| Mercury Developments Limited | Construction | 1999 |
| Mercury Management Limited | Households | 1982 |
| Mercy and Mercy Distribution Limited | Hospitality | 2026 |
| Meridian Accountants and Consultants Limited | Professional & scientific | 2013 |
| Merkay Limited | Wholesale & retail | 2021 |
| Merton Acquisitions Limited | Construction | 2016 |
| Merton Catalyst LLP | | 2018 |
| Mesana Properties Limited | Real estate | 1998 |
| Message Fresh Limited | Arts & recreation | 2010 |
| Meta Pedagogy Limited | Information & communication | 2025 |
| Metisphere Ltd | Information & communication | 2021 |
| Metropolitan Electricians Ltd | Construction | 2025 |