| Martin Cross Media Limited | Information & communication | 2020 |
| Martins Marine Engineering UK Ltd | Manufacturing | 2010 |
| Martmal Ltd | Professional & scientific | 2022 |
| Marvel Creative Design Limited | Arts & recreation | 2014 |
| Marwal Consultancy Services Ltd | Professional & scientific | 2025 |
| Marzipan Clothing Limited | Manufacturing | 2025 |
| Masachem Ltd | Administrative & support | 2022 |
| Masefield Gardens Management Company Limited | Households | 1997 |
| Masefield Gardens Residents Association Limited | Households | 1997 |
| Mass Holdings Limited | Professional & scientific | 1993 |
| Mass Information Systems Limited | Information & communication | 1991 |
| Master Design and Build Ltd | Construction | 2020 |
| Master Plastering Reading Limited | Construction | 2021 |
| Matlary Ltd | Hospitality | 2020 |
| Matopos Ltd | Professional & scientific | 2025 |
| Matriarch Studios Ltd | Professional & scientific | 2025 |
| Matt Lamplugh Design Ltd | Professional & scientific | 2020 |
| Matt Perry Limited | Arts & recreation | 2014 |
| Matthew James Painting & Decorating Limited | Construction | 2021 |
| Matthew M Property's Ltd | Real estate | 2025 |
| Maverick Northbrook Ltd | Real estate | 2026 |
| MAX Plumbing & Heating Ltd | Construction | 2019 |
| Maxdale Consulting Limited | Information & communication | 2018 |
| Maxfe Limited | Wholesale & retail | 2003 |
| Maximillian LDN Limited | Information & communication | 2023 |
| Maxon Motor UK Ltd | Manufacturing | 1999 |
| Maxwell Felix Limited | Administrative & support | 2011 |
| MAY Block Management Ltd | Households | 2026 |
| MAY Fields Guest House Limited | Hospitality | 2014 |
| Maya Art&Decoration Ltd | Manufacturing | 2023 |
| Maybin Electrical Ltd | Construction | 2013 |
| Maybrook Gardens Residents Association Limited | Households | 1981 |
| Maypen Care Limited | Health & social care | 2022 |
| MB Didcot Limited | Hospitality | 2025 |
| MB East Kilbride Limited | Hospitality | 2025 |
| MB Glasgow Limited | Hospitality | 2025 |
| MC Seven Real Estate Ltd | Real estate | 2025 |
| MCC Motors Ltd | Wholesale & retail | 2017 |
| Mccski Limited | Wholesale & retail | 2024 |
| Mcdonald Intellectual Property Limited | Professional & scientific | 2016 |
| Mcginnis LOY Associates Ltd | Administrative & support | 2012 |
| MCJ Building Services Limited | Construction | 2023 |
| MD Creative Ltd | Administrative & support | 2020 |
| MD Growth Consulting Ltd | Professional & scientific | 2024 |
| Mdak Ltd | Real estate | 2014 |
| Mdsg Construction Ltd | Construction | 2026 |
| MDV Consulting Limited | Professional & scientific | 2014 |
| MDW 48 Limited | Information & communication | 2020 |
| ME & YOU Productions Ltd | Information & communication | 2012 |
| ME Industries Limited | Administrative & support | 2003 |
| Meadow Grange (Downton) Management Company Limited | Households | 2022 |
| Meadow Mart Ltd | Wholesale & retail | 2020 |
| MEB Building & Refurbishment Limited | Construction | 2002 |
| Meddydent Limited | Manufacturing | 2015 |
| Media Call Ltd | Information & communication | 2022 |
| Mediakeys Ltd | Professional & scientific | 2018 |
| Medic on Hand Ltd | Education | 2022 |
| Medicals Direct Screenings Limited | Health & social care | 1992 |
| Medicals Direct Screenings Services Limited | Financial services | 2018 |
| Meditor Capital Management Limited | Financial services | 1998 |
| Medlist International Limited | Other services | 2018 |
| Meet the Street Ltd | Administrative & support | 2026 |
| Megan Ingham Dressage Ltd | Agriculture & fishing | 2024 |
| MEI Dome Ltd | Real estate | 2024 |
| MEL Hampshire Voiceovers Ltd | Information & communication | 2019 |
| Melanie Curtis Accountants Ltd | Professional & scientific | 2004 |
| Melmac Solutions Limited | Extraterritorial | 1995 |
| Melrose Court Owners Limited | Households | 2012 |
| MEM (Plumbing & Electrical) Services Ltd | Construction | 2014 |
| Menopause Health Ltd | Health & social care | 2017 |
| Mention Limited | Professional & scientific | 2007 |
| Merark Solutions Limited | Information & communication | 2026 |
| Mercatoluxe Ltd | Wholesale & retail | 2026 |
| Merge Visualisation Studios Ltd | Information & communication | 2015 |
| Merit Flooring Limited | Construction | 2022 |
| Mertonford (Wokingham) Limited | Hospitality | 1988 |
| Mesha Group Limited | Information & communication | 2025 |
| Mesoskinline (UK) Limited | Wholesale & retail | 2019 |
| Meta-Ethic Security Ltd | Real estate | 2020 |
| Methodic Homes Ltd | Construction | 2025 |
| Metis Project Management Limited | Professional & scientific | 2019 |
| Metispeople Limited | Administrative & support | 2014 |
| Metisplan Limited | Professional & scientific | 2012 |
| Metisproperties Limited | Real estate | 2019 |
| Metro Digital Systems Limited | Information & communication | 1999 |
| Metro Digital Television Limited | Information & communication | 1992 |
| Metro Group Limited | Professional & scientific | 1995 |
| Metro Industrial Centre Management Limited | Households | 1986 |
| Metropolis Studio Ltd | Construction | 2019 |
| Mfarrell Tech Consulting Ltd | Information & communication | 2026 |
| Mfintech Ltd | Information & communication | 2022 |
| MG Auto Locks Ltd | Professional & scientific | 2024 |
| MG Chauffeurs Ltd | Transport & logistics | 2021 |
| MG Energy and Engineering Limited | Professional & scientific | 2025 |
| MGI Global Ltd. | Health & social care | 2024 |
| MGK Consultant Limited | Information & communication | 2020 |
| MH Bespoke Interiors Ltd | Manufacturing | 2025 |
| MH Motor Experts Ltd | Wholesale & retail | 2012 |
| MHA Estates Ltd | Real estate | 2017 |
| MHV Services Limited | Administrative & support | 2013 |