| Marr Project Services Ltd | Professional & scientific | 2021 |
| Marriott Investments Ltd | Real estate | 2020 |
| Mars Fast Ltd | Wholesale & retail | 2024 |
| Marsh Plumbing Services Ltd | Construction | 2019 |
| Marsh Solutions Limited | Professional & scientific | 2020 |
| Marshall's Corporation Limited | Real estate | 2017 |
| Marshall's Group Holdings Limited | Financial services | 2025 |
| Marshall's Logistics Limited | Transport & logistics | 2016 |
| Marshcall Ltd. | Real estate | 2009 |
| Marsil Marine Group Limited | Professional & scientific | 1997 |
| Marsil Marine Limited | Transport & logistics | 1983 |
| Martco Consultants Ltd | Real estate | 2022 |
| Martial-Arts in Education Ltd | Education | 2023 |
| Martin Elliott & Co Limited | Professional & scientific | 2017 |
| Martin Goss Consulting Limited | Information & communication | 2024 |
| Martin Roche Consultancy Limited | Other services | 2017 |
| Martin Scott Properties Limited | Administrative & support | 2008 |
| Martin Watts Property Limited | Financial services | 2023 |
| Martin's Trans Ltd | Construction | 2019 |
| Martintrux Group Limited | Transport & logistics | 1982 |
| Martus Limited | Wholesale & retail | 2018 |
| Marver Publicity Limited | Professional & scientific | 2001 |
| Mary Anne Fedeyko Limited | Professional & scientific | 2006 |
| Marzbani Oralcare Ltd | Health & social care | 2025 |
| MAS Innovations Ltd | Wholesale & retail | 2025 |
| Masdale Limited | Households | 1994 |
| Mashdel Services Ltd | Health & social care | 2014 |
| Maska Limited | Health & social care | 2001 |
| Mason Consulting Services Limited | Other services | 2017 |
| Mason Rail Limited | Transport & logistics | 2014 |
| Mason Trucking Co Ltd | Transport & logistics | 2016 |
| Masons Tree Surgery Limited | Other services | 2004 |
| Masque Studio Limited | Arts & recreation | 2013 |
| Mass Surveys Limited | Professional & scientific | 2008 |
| Massive Passive Ltd | Real estate | 2021 |
| Master Coach Ltd | Education | 2022 |
| Master Finishing Crew Ltd | Construction | 2017 |
| Mastershield Roofing & Building Services Limited | Construction | 2019 |
| Masti Lounge Ltd | Hospitality | 2023 |
| Mastic Services Limited | Construction | 2016 |
| Mata Pathibhara Ltd | Real estate | 2025 |
| Matane Sushi Colchester Ltd | Hospitality | 2021 |
| Matchplay Solutions Ltd | Construction | 2026 |
| Mather Theatre Arts Ltd | Arts & recreation | 2025 |
| Mathi Investments Ltd | Real estate | 2018 |
| Mati & Nanda Ltd | Wholesale & retail | 2019 |
| Matonok Consulting Ltd | Administrative & support | 2019 |
| Matopos Spirit Ltd | Manufacturing | 2020 |
| Matrix Fitness Store Limited | Wholesale & retail | 2013 |
| Matrix Marketing Consultancy Ltd | Professional & scientific | 1999 |
| Matrix Motors Limited | Wholesale & retail | 2019 |
| Matrix Underwriting Management Ltd | Financial services | 2010 |
| Matt Inc. Limited | Administrative & support | 2000 |
| Matt Lacey Limited | Information & communication | 2011 |
| Matt Minnis Builders Limited | Construction | 2021 |
| Matt Plant Ltd | Manufacturing | 2010 |
| Matthew Ford Food Ltd | Hospitality | 2014 |
| Matthew James Property Consultants Limited | Real estate | 2018 |
| Matthew James Property Group Ltd | Construction | 2018 |
| Matthews and Sons Maintenance Ltd | Construction | 2024 |
| Mattstic Sealants Ltd | Manufacturing | 2018 |
| Maturecourt Builders Limited | Construction | 1982 |
| Maunsell Park Crawley (1) Management Company Limited | Households | 1990 |
| Maunsell Park Crawley (2) Management Company Limited | Households | 1990 |
| Maureen Saville Limited | Other services | 2006 |
| Mauritian Nutrition Ltd | Hospitality | 2020 |
| Maviton Limited | Real estate | 1964 |
| Mawer Psychology Services Ltd | Professional & scientific | 2023 |
| MAX Clad Ltd | Construction | 2023 |
| MAX Competitions Ltd | Wholesale & retail | 2025 |
| MAX Emerald Ltd | Professional & scientific | 2020 |
| MAX H.R. Consulting Limited | Administrative & support | 2002 |
| MAX Lettings Ltd | Real estate | 2024 |
| MAX Property Finance UK Limited | Financial services | 2022 |
| MAX Scaffolding Ltd | Construction | 2020 |
| MAX the Magician Ltd | Wholesale & retail | 2024 |
| MAX the Magician Trading Ltd | Wholesale & retail | 2024 |
| Maxdealz Limited | Wholesale & retail | 2026 |
| Maxi Print Limited | Manufacturing | 2022 |
| Maxitech Turbos Limited | Administrative & support | 1998 |
| Maxwell Wobidike Construction Ltd | Construction | 2021 |
| Maxwood Consultancy Ltd | Administrative & support | 2016 |
| Maxximus Ltd | Other services | 2025 |
| MAY Consult Limited | Administrative & support | 2018 |
| MAY MAY (2017) Ltd | Wholesale & retail | 2017 |
| May&Roy Ltd | Transport & logistics | 2016 |
| Maya Tamila Properties UK Ltd | Real estate | 2023 |
| Maya-Carla Construction Ltd | Construction | 2021 |
| Maydays Estate Services Limited | Construction | 2003 |
| Maydays Farms Ltd | Agriculture & fishing | 2020 |
| Maydene Ltd | Agriculture & fishing | 2017 |
| Mayfair Online Group Ltd | Wholesale & retail | 2022 |
| Mayfield ES Limited | Construction | 2018 |
| Mayflower 888 Ltd | Hospitality | 2024 |
| Mayflower Films Limited | Information & communication | 2014 |
| Mayflower Flat Management Ltd | Households | 2009 |
| Maylad Support and Rehabilitation Limited | Health & social care | 2026 |
| Mayland Interiors Ltd | Construction | 2019 |
| Mayland Manufacturing Ltd | Manufacturing | 2021 |
| Mayo Traders Ltd | Wholesale & retail | 2023 |