| Milcom Limited | Information & communication | 2005 |
| Milgate Properties Limited | Construction | 2003 |
| Millbank Life & Pensions Limited | Financial services | 1994 |
| Millbank Property Investments Limited | Real estate | 1999 |
| Miller Rose Limited | Real estate | 2018 |
| Millfields Developments Limited | Construction | 2006 |
| Millsted Holdings Ltd | Professional & scientific | 2020 |
| Milmal Limited | Professional & scientific | 2003 |
| Milners Limited | Wholesale & retail | 2008 |
| Milton & Co UK Limited | Professional & scientific | 1999 |
| Mimosa Limited | Real estate | 1994 |
| Mind-Fit Strategies Ltd | Professional & scientific | 2003 |
| Mindful Learning Limited | Education | 2026 |
| Minnesota Investment Limited | Financial services | 1984 |
| Minorca Sailing Holidays Limited | Administrative & support | 1978 |
| Minster Care Group Limited | Health & social care | 2017 |
| Minster Care Management Limited | Health & social care | 1998 |
| Mint Medical Services Ltd | Professional & scientific | 2017 |
| Mintway Limited | | 1985 |
| Miraya Property Service Limited | Real estate | 2021 |
| Miscarriage Clinic Limited | Health & social care | 2009 |
| Miscellanea Developments Limited | Wholesale & retail | 2004 |
| Misfits Productions Limited | Administrative & support | 2002 |
| Misoplus Ltd | Wholesale & retail | 2022 |
| Missing Link Solutions Limited | Information & communication | 1997 |
| Mistral Computer Technology Limited | | 1991 |
| Mitcham Estates Limited | Professional & scientific | 2001 |
| Mitcham Foods Centre Ltd | Hospitality | 2025 |
| Mitchell Finishing Limited | Wholesale & retail | 1985 |
| Mitox Limited | Information & communication | 2004 |
| Mitrerose Limited | Wholesale & retail | 1979 |
| MIX360 Ltd | Information & communication | 2005 |
| Mixmicro Data Limited | Information & communication | 1989 |
| MJF Specialist Decor Limited | Construction | 2003 |
| MJH Holdings Limited | Financial services | 2002 |
| MJT Plumbing and Heating Limited | Construction | 2006 |
| Mksg Consulting Ltd | Information & communication | 2016 |
| MKW Limited | Transport & logistics | 2004 |
| MLM Strategy Consulting Limited | Professional & scientific | 1997 |
| MM Executive Protection Ltd | Administrative & support | 2025 |
| MMG Accountancy Services Limited | Transport & logistics | 2002 |
| MNF Oven Ltd | Hospitality | 2017 |
| Mo-Id Ltd | Professional & scientific | 2014 |
| Mobile Mania Ltd | Administrative & support | 2018 |
| Mobiquitis Ltd | Professional & scientific | 2007 |
| Mobruby Ltd | Health & social care | 2016 |
| Modern Astronomy Ltd | Wholesale & retail | 2005 |
| Modern Classics Direct Limited | Transport & logistics | 2006 |
| Modern Colours Limited | Administrative & support | 2003 |
| Modern Developments 2018 Limited | Real estate | 2018 |
| Molesey Refrigeration Limited | Real estate | 2008 |
| Molini Limited | Information & communication | 2005 |
| Moma Investments Ltd | Real estate | 2022 |
| Monar Engineering Services Limited | Construction | 1994 |
| Monarchs Gate Residents Company Limited | Households | 1999 |
| Moneyandmeclaims Limited | Administrative & support | 2009 |
| Moneyiq Ltd | Professional & scientific | 1999 |
| Monmart Limited | Professional & scientific | 1993 |
| Montago Accounting Ltd | Professional & scientific | 2007 |
| Montagu Renovations Limited | Construction | 2007 |
| Montrose Building Services Limited | Construction | 1980 |
| Monza Software Limited | Information & communication | 1997 |
| Moody & Associates Limited | Professional & scientific | 2007 |
| Moonlight Antiques Limited | Wholesale & retail | 2025 |
| Moonlight Internet Services Limited | Information & communication | 1997 |
| Moonstone Press Ltd | Administrative & support | 2016 |
| Moonstone Travel Limited | Professional & scientific | 1995 |
| Moonwhizz Ltd | Wholesale & retail | 1999 |
| Moor Park Capital Next GEN Ltd | Real estate | 2023 |
| Moores' Cleaning & Decorating Services Limited | Construction | 2005 |
| Moorlands (Lightwater) Management Company Limited | Households | 1989 |
| Mordec (UK) Limited | Construction | 2006 |
| Morden News Ltd | Wholesale & retail | 2015 |
| Morgan and Small Ltd | Other services | 1953 |
| Morgan, Adler & Co | | 1993 |
| Morris Leslie South East Limited | Administrative & support | 2005 |
| Morse Wedding Services Limited | Other services | 2000 |
| Mosaic Point Limited | Real estate | 2023 |
| Mossley Limited | Households | 1996 |
| Motshop UK Limited | Other services | 2003 |
| Motus Limited | Information & communication | 1998 |
| Mountain Greenery Limited | Professional & scientific | 2001 |
| Mountford Pigott LLP | | 2007 |
| Move Revolution EOT Limited | Administrative & support | 2023 |
| Movecentral Property Management Limited | Other services | 1989 |
| Movement Matters Limited | Professional & scientific | 1998 |
| Mowtivated Holdings Limited | Financial services | 2024 |
| MPB Associates Ltd. | Hospitality | 2001 |
| MPH Creative Solutions Limited | Professional & scientific | 2006 |
| MPS Pharmacy Limited | Wholesale & retail | 2003 |
| Mqube Technology Ltd | Information & communication | 2023 |
| MR Fixer Ltd | Construction | 2007 |
| MR Traders London Ltd | Wholesale & retail | 2017 |
| MR. Carpet Limited | Wholesale & retail | 1987 |
| MRC Associates Limited | Professional & scientific | 2007 |
| MS Head Ltd | Other services | 2022 |
| MS Paula Bear Ltd | Education | 2024 |
| MSA Accounting & Taxation Services Limited | Professional & scientific | 2005 |
| MSI Commerce Limited | Information & communication | 2007 |
| MSK Solutions Limited | Information & communication | 2000 |