| Mabelese Limited | Professional & scientific | 2012 |
| MAD Systems Limited | Other services | 2012 |
| Maee Ltd | Information & communication | 2023 |
| Magenta Fine Art Limited | Wholesale & retail | 2021 |
| Magi Investments Limited | Real estate | 2016 |
| Magic by Post Ltd | Wholesale & retail | 2025 |
| Magma Ironing Services Ltd | Other services | 2025 |
| MAI Productions Ltd | Professional & scientific | 2019 |
| Maia Collective Ltd | Professional & scientific | 2025 |
| Maidenhead Independent Limited | Financial services | 1995 |
| Maintenance Associated Services Limited | Administrative & support | 1995 |
| Mairead Quinn Communications Ltd | Other services | 2013 |
| Maison Moire Limited | Professional & scientific | 2019 |
| Mall Properties Ltd | Real estate | 2014 |
| Malnen Properties Limited | Real estate | 2019 |
| Maltings Pharma (Holdings) Ltd | Financial services | 2016 |
| Maltings Pharmacy Limited | Wholesale & retail | 2002 |
| Mandamore Limited | Real estate | 2022 |
| Mani Construction Ltd | Construction | 2021 |
| Manid Limited | Health & social care | 2013 |
| Manolete Partners PLC | Administrative & support | 2011 |
| Manor Lane Management Limited | Administrative & support | 1973 |
| Maple House Holdings Limited | Financial services | 2022 |
| Maple Partnership Limited | Professional & scientific | 2022 |
| Mapleroots Limited | Construction | 2025 |
| Marburn Limited | Financial services | 2007 |
| Marcus Domleo Limited | Information & communication | 2012 |
| Maribel Communications Ltd. | Information & communication | 1994 |
| Marie Cavanagh Limited | Information & communication | 2011 |
| Marika Properties Ltd | Real estate | 2025 |
| Marillier Consulting Limited | Professional & scientific | 2020 |
| Mark Rayner Finance Limited | Professional & scientific | 2014 |
| Mark Roberts Special Effects Limited | Other services | 2018 |
| Marketing Force Limited | Professional & scientific | 1999 |
| Markettplace.Com Ltd | Wholesale & retail | 2020 |
| Marlborough Sports Media Ltd | Professional & scientific | 2013 |
| Maroon Films Ltd | Information & communication | 2014 |
| Marsham Capital Ltd | Real estate | 2023 |
| Marsham Studio Ltd | Professional & scientific | 2023 |
| Marsham Ventures Ltd | Professional & scientific | 2018 |
| Martin James (London) Limited | Wholesale & retail | 2003 |
| Martin James London (Retail) Limited | Wholesale & retail | 2008 |
| Martoni Investments Ltd | Financial services | 2026 |
| Marula Africa Mining Holdings Ltd | Mining & quarrying | 2025 |
| Marula Metals Processing Ltd | Mining & quarrying | 2025 |
| Marula Mining PLC | Mining & quarrying | 2001 |
| Marylebone Mews Estates Limited | Real estate | 2024 |
| MAS One Limited | Real estate | 2024 |
| Mascioniassociati International Limited | Professional & scientific | 2009 |
| Maskelyne Limited | Real estate | 2016 |
| Masons Plumbing & Heating Supplies Limited | Wholesale & retail | 1995 |
| Massey Blick Consulting Ltd | Administrative & support | 2018 |
| Massot Limited | Professional & scientific | 2015 |
| Masstech Limited | Professional & scientific | 1992 |
| Master Grooming Ltd | Other services | 2024 |
| Master Key Solutions Limited | Other services | 2004 |
| Masterkod Software and Consulting Ltd | Information & communication | 2020 |
| Matamo Financial Services Ltd | Financial services | 2023 |
| Matich Technical Services Ltd | Professional & scientific | 2024 |
| Matthew Childs Design Limited | Administrative & support | 2015 |
| Matthew Grace Delivery Limited | Professional & scientific | 2011 |
| Matthews Yardley Limited | Administrative & support | 2014 |
| Maude Hirst Yoga Ltd | Education | 2018 |
| Maund AI Limited | Information & communication | 2026 |
| Mauritius NOW Limited | Information & communication | 2009 |
| MAV Properties (UK) Ltd | Construction | 2006 |
| Maverick Foodworks Ltd | Hospitality | 2025 |
| Maximizer Software Limited | Information & communication | 1985 |
| Maxlotte Sports Limited | Manufacturing | 2026 |
| Maxtian Interim Limited | Professional & scientific | 2004 |
| Maxtian Limited | Professional & scientific | 2002 |
| MAY Trees Management Company Limited | Administrative & support | 2001 |
| Mayfair Global Partnership Limited | Other services | 2017 |
| Mayfair Pharmacare Limited | Wholesale & retail | 2010 |
| Mayflower Barn Properties Limited | Real estate | 2021 |
| Mayhew Contracting Limited | Other services | 2010 |
| Maywind Estates Limited | Households | 2018 |
| Maywind Medical Limited | Health & social care | 2021 |
| MBM Property Ventures Limited | Real estate | 2019 |
| MBM Ventures Limited | Real estate | 2012 |
| Mccready Medical LLP | | 2020 |
| Mcgill Music SAX School Limited | Education | 2014 |
| Mclean Talent Group Limited | Professional & scientific | 2025 |
| Mcthurston Property Ltd | Hospitality | 2026 |
| MD & Associates Holdings Ltd | Real estate | 2023 |
| MD Communications Limited | Professional & scientific | 2010 |
| MDW Solicitors Ltd | Professional & scientific | 2019 |
| MED Stop Limited | Wholesale & retail | 2017 |
| Medflicks Ltd | Health & social care | 2023 |
| Media Safety Limited | Professional & scientific | 2003 |
| Medicalease Ltd | Information & communication | 2022 |
| Medication Masters Limited | Professional & scientific | 2020 |
| Medway Property Developments Ltd | Construction | 2015 |
| Mehtawise Ltd | Professional & scientific | 2022 |
| MEL Consultancy Ltd | Information & communication | 2022 |
| Meme TV Limited | Information & communication | 2013 |
| Menesse Barbers Ltd | Other services | 2022 |
| Meridian Capital (GB) Ltd | Financial services | 2009 |
| Merimed (Harley Street) Ltd | Health & social care | 2020 |
| Messias Properties LLP | | 2022 |