| Luxford and Ward Ltd | Real estate | 2024 |
| Lynde Lodge Management Limited | Households | 2025 |
| Lytton Heights (Rabley Heath) Management Company Limited | Households | 2019 |
| M J Danaher Ltd | Health & social care | 2015 |
| M J Rice Financial Services Limited | Professional & scientific | 2007 |
| M M Impex Limited | Other services | 2020 |
| M Stachini Ltd | Information & communication | 2018 |
| M T Wilson Motor Repairs Limited | Wholesale & retail | 2003 |
| M63 Consulting Limited | Professional & scientific | 2024 |
| Macbreck Limited | Real estate | 2008 |
| Madjem Limited | Financial services | 2011 |
| Maedean Ltd | Construction | 2021 |
| Magicautos Ltd | Construction | 2024 |
| Make YOU Smile Limited | Public administration | 2015 |
| Mama Quina Ltd | Hospitality | 2022 |
| Mamara Limited | Professional & scientific | 2017 |
| Managed Print AS a Service Ltd | Administrative & support | 2026 |
| Management 50 Ltd | Households | 2026 |
| Manly Investments Limited | Construction | 1992 |
| Mardley Electrical Limited | Construction | 2019 |
| Mardley Health and Investments Limited | Real estate | 2022 |
| Mark Tempest Autocentre Limited | Wholesale & retail | 1999 |
| Marks Lending Limited | Financial services | 2013 |
| Marks Property Limited | Real estate | 2013 |
| Marlborough Residents Association Limited | Real estate | 2005 |
| Marpaul Trading Limited | Other services | 2014 |
| Martial Mind Power Ltd | Information & communication | 2024 |
| Martin Williams Consultancy Ltd | Professional & scientific | 2016 |
| Medcare Solutions Ltd | Health & social care | 2025 |
| Megatron Steels (Holdings) Limited | Professional & scientific | 2016 |
| Megatron Steels Limited | Manufacturing | 2016 |
| MEL Media World Ltd | Wholesale & retail | 2013 |
| Mepod Limited | Manufacturing | 2024 |
| Metro Estates (Welwyn) Limited | Real estate | 1970 |
| Metrology & Quality Services Limited | Professional & scientific | 1998 |
| MG Construction Solutions Ltd | Construction | 2019 |
| Michael Iter Lumine CIC | Professional & scientific | 2020 |
| Micks Ware Ltd | Other services | 2025 |
| Midgley Homes Limited | | 1984 |
| Mike Ryder Automotive Ltd | Wholesale & retail | 2025 |
| Mills Pearson Limited | Real estate | 1976 |
| Mimram Homes Ltd | Real estate | 2024 |
| Mimram Place Management Limited | Households | 1994 |
| Mimram Studios Limited | Professional & scientific | 2019 |
| Mimram Valley Laundry Limited | Other services | 2016 |
| Mindshare Consulting Limited | Information & communication | 2007 |
| Minicoy Developments Limited | Real estate | 2015 |
| Minicoy Investments (Stevenage) Limited | Real estate | 2022 |
| Minicoy Investments Limited | Real estate | 2015 |
| Mistry Construction Ltd | Construction | 2021 |
| Mivel's & Co. Ltd | Financial services | 2014 |
| Mivels Financial & Management Ltd. | Professional & scientific | 2020 |
| Mjekiqi HR Ltd | Professional & scientific | 2022 |
| Mjekiqi Property Investment Ltd | Real estate | 2020 |
| MKS Advisors Ltd | Professional & scientific | 2014 |
| Mmrs PD Limited | Professional & scientific | 2023 |
| Mnic Homes Ltd | Construction | 2025 |
| Mnic Properties Ltd | Construction | 2017 |
| Mobilistic Marketing Ltd | Professional & scientific | 2007 |
| Monday Swimwear Limited | Wholesale & retail | 2018 |
| Money Orchard Limited | Financial services | 2022 |
| Mont Arch Ltd | Construction | 2023 |
| Moons Toyland Limited | Wholesale & retail | 1991 |
| MOR Cliff Management Company Limited | Real estate | 2012 |
| Moto TOUR1 Limited | Administrative & support | 2014 |
| Motorsport Draft League Limited | Administrative & support | 2025 |
| MPS Colourprint Limited | Professional & scientific | 2008 |
| MR & MRS Retro SOX Ltd | Wholesale & retail | 2021 |
| MT Construction Leadership Limited | Professional & scientific | 2024 |
| Mtronix Limited | Wholesale & retail | 2025 |
| MV Healthcare Consulting Limited | Professional & scientific | 2010 |
| My Vintage Market Ltd | Wholesale & retail | 2019 |
| Myrobotaxi Ltd | Transport & logistics | 2025 |
| N & GVB Consulting Limited | Information & communication | 2013 |
| N.D.E.X Ltd | Professional & scientific | 2017 |
| Naanak Holdings Ltd | Other services | 2017 |
| Nara Brand Ltd | Professional & scientific | 2025 |
| Natural Toothpaste Limited | Wholesale & retail | 2024 |
| NDC Consulting Ltd | Professional & scientific | 2022 |
| Necto Community Interest Company | Administrative & support | 2018 |
| Nemus Limited | Professional & scientific | 2024 |
| Neonetra Ltd | Information & communication | 2025 |
| Netcel Communications Limited | Hospitality | 1996 |
| Nevill Group Ltd | Construction | 2022 |
| New Dawn Care Group Ltd | Health & social care | 2025 |
| New HAW Barber Ltd | Other services | 2020 |
| New Place (Welwyn) Residents Association 1983 Limited | Households | 1983 |
| Newbury Wood Limited | Administrative & support | 2007 |
| Newera Computing Limited | Extraterritorial | 1987 |
| Nexttech Software Ltd | Information & communication | 2025 |
| NFB Transport Systems AB | | 1956 |
| Nfbi Limited | Real estate | 2003 |
| Nick Allsop Ltd | Administrative & support | 2024 |
| Nick Dansie Locums Ltd | Health & social care | 2023 |
| Non-Stop Coffee Shop Limited | Hospitality | 2022 |
| Norfolk Capsey Ltd | Administrative & support | 2025 |
| Noslien Consulting Ltd | Extraterritorial | 2018 |
| NOT Another PEN Ltd | Wholesale & retail | 2019 |
| Nova Methods Limited | Information & communication | 2024 |
| Novabio-International Limited | Professional & scientific | 2026 |