| Marios UK Limited | Construction | 2009 |
| Maritrans Limited | Transport & logistics | 2018 |
| Marium Akram Ltd | Health & social care | 2026 |
| Marius & Lili Limited | Construction | 2019 |
| Marius Carpin Ltd | Transport & logistics | 2020 |
| Marius&Maria Ltd | Other services | 2018 |
| Mariusbeciu Limited | Construction | 2017 |
| Mariusz Kozlowski Ltd | Transport & logistics | 2022 |
| Mark 7 Property Limited | Construction | 2021 |
| Mark Bird Flooring Limited | Wholesale & retail | 2002 |
| Mark Booth Prosthetics Technicians Limited | Health & social care | 2014 |
| Mark Buller Shoe Agencies Ltd | Other services | 2022 |
| Mark Darling Eyecare Ltd | Wholesale & retail | 2009 |
| Mark Elliott Associates Limited | Construction | 2008 |
| Mark Goatley Limited | Agriculture & fishing | 2003 |
| Mark Goodger Antiques Ltd | Wholesale & retail | 2024 |
| Mark Lead IT Limited | Information & communication | 2024 |
| Mark Richardson Consultants Limited | Professional & scientific | 2006 |
| Mark Spatcher Lettings Ltd | Real estate | 2021 |
| Mark Spatcher Ltd | Construction | 2020 |
| Mark Wolstenholme Ltd | Transport & logistics | 2020 |
| Markanto Design and Build Ltd | Construction | 2021 |
| Market Place Accounting Services (Northampton) LLP | | 2010 |
| Market Place Accounting Services LLP | | 2010 |
| Marketing by CPC Limited | Professional & scientific | 2024 |
| Marketing Infusions Ltd | Professional & scientific | 2015 |
| Markjan Investments Limited | Real estate | 2019 |
| Marko Ltd | Transport & logistics | 2012 |
| Marktrans Ltd | Transport & logistics | 2011 |
| Marla Fire Ltd | Construction | 2017 |
| Marloe & Birch Ltd | Wholesale & retail | 2025 |
| Marlow 2 Investments Ltd | Financial services | 2019 |
| Marlton Enterprises Limited | Administrative & support | 2026 |
| Marmalade Communications Limited | Education | 2024 |
| Marmar Trans Ltd | Transport & logistics | 2015 |
| Marmaris BBQ Ltd | Hospitality | 2017 |
| Marmite Communications Consultancy Ltd | Professional & scientific | 2025 |
| Marnicam Limited | Real estate | 2025 |
| Marnie Construction Limited | Construction | 2018 |
| Marnie Properties Limited | Real estate | 2023 |
| Marodach Limited | Construction | 2024 |
| Maronmarch Limited | Wholesale & retail | 1999 |
| Marpik Driving Academy Ltd | Information & communication | 2023 |
| Marquees of Woburn Ltd | Administrative & support | 2021 |
| Marquis Rhinefield Ltd | Wholesale & retail | 2024 |
| Marragone Flat Management Company Limited | Households | 1978 |
| Marren Business Systems Limited | Extraterritorial | 1997 |
| Marren Microwave Limited | Manufacturing | 1985 |
| Marriage Allowance Tax Rebate Limited | Information & communication | 2019 |
| Marriya & Zahraa Property Investments Limited | Real estate | 2018 |
| Marrowell Farm Services Limited | Wholesale & retail | 1977 |
| Marrum & Co Ltd | Financial services | 2024 |
| Marrum Distribution Ltd | Wholesale & retail | 2013 |
| Marsden (Construction) Ltd | Construction | 2011 |
| Marsden Fire Safety Limited | Wholesale & retail | 2004 |
| Marshall & Ashcroft Property Management Ltd | Real estate | 2022 |
| Marshall Real Estate Investment Limited | Real estate | 2021 |
| Marshall Sadler Ltd | Construction | 2019 |
| Marshall Wallace Limited | Transport & logistics | 1958 |
| Marshall's MMA LLP | | 2017 |
| Marshall’S Roofing Services Ltd | Construction | 2025 |
| Marsi Ltd | Transport & logistics | 2026 |
| Marston Building and Property Limited | Real estate | 2021 |
| Marston Park (Marston Moretaine) Management Company Limited | Households | 2011 |
| Martel Properties Limited | Real estate | 2022 |
| Martex Hero Ltd | Wholesale & retail | 2024 |
| Martin & Transport Ltd | Transport & logistics | 2014 |
| Martin Burrows Plumbers Limited | Construction | 2003 |
| Martin Clarke Associates Limited | Professional & scientific | 2016 |
| Martin Conway Limited | Health & social care | 2010 |
| Martin COX Chamois Finishers Limited | Wholesale & retail | 2003 |
| Martin Fire Risk Management Limited | Professional & scientific | 2015 |
| Martin Henry Financial Services Limited | Professional & scientific | 2016 |
| Martin HGV Ltd | Administrative & support | 2016 |
| Martin Properties (Midlands) Ltd | Real estate | 2025 |
| Martin Sale Ltd | Wholesale & retail | 2014 |
| Martin Smith Solutions Limited | Wholesale & retail | 2026 |
| Martin Sussex Medical Limited | Other services | 2022 |
| Martindale Windows and Developments Limited | Construction | 2011 |
| Maruthi Investments Ltd | Real estate | 2022 |
| Marx Ayur Limited | Other services | 2022 |
| Marx Ayur Walsall Limited | Other services | 2025 |
| Marylebone Warwick Balfour Management Limited | Construction | 1994 |
| Marysia Services Ltd | Transport & logistics | 2023 |
| Maryvera's Home Limited | Health & social care | 2025 |
| MAS Investment Group Ltd | Real estate | 2017 |
| MAS Pubs Ltd | Hospitality | 2026 |
| MAS Rail Services Ltd | Construction | 2012 |
| Masasana Ltd | Professional & scientific | 2016 |
| Mascons Group Ltd | Construction | 2018 |
| Mascote Renovations Ltd | Construction | 2024 |
| Mase Advisory Ltd | Administrative & support | 2026 |
| Maselli UK Limited | Wholesale & retail | 2011 |
| Mashange Services Ltd | Health & social care | 2022 |
| Masih Devitt & Associates Ltd | Professional & scientific | 2021 |
| Masimo Delivery Ltd | Transport & logistics | 2025 |
| Masis Medical Ltd | Health & social care | 2025 |
| Mask Northampton Limited | Other services | 2006 |
| Mask OFF C.I.C. | Health & social care | 2022 |
| Mask Plumbing Solutions Ltd | Construction | 2022 |