| Marriott Vernon Limited | Real estate | 2019 |
| Marrone Service UK Ltd | Manufacturing | 2026 |
| Mars Global Limited | Wholesale & retail | 2010 |
| Marsberthy Ltd | Real estate | 2024 |
| Marsh Farm Management Company Limited | Households | 2010 |
| Marsh House Stafford RTM Company Limited | Households | 2011 |
| Marsh Vernacular Ltd | Real estate | 2023 |
| Marshall Hashimi Limited | Construction | 2025 |
| Marshall Lawrence Limited | Real estate | 2018 |
| Marston Fields (Property Management) Company NO. 1 Limited | Real estate | 2003 |
| Marston Fields (Property Management) Company NO.2 Limited | Households | 2003 |
| Martaswell Ltd | Wholesale & retail | 2018 |
| Martek Contracts Limited | Construction | 1982 |
| Martek Design & Display Limited | Construction | 1993 |
| Martek Joinery Limited | Construction | 1993 |
| Martek Ltd | Manufacturing | 2023 |
| Martek Zero Waste Ltd | Manufacturing | 2022 |
| Martello Management (No's 20-22) Limited | Households | 2007 |
| Martfede Ltd | Hospitality | 2022 |
| Marti Services Ltd | Construction | 2016 |
| Martil Ltd | Hospitality | 2019 |
| MARTIL2CAFE Ltd | Hospitality | 2025 |
| Martin Renovations Ltd | Construction | 2014 |
| Martinelle Ltd | Wholesale & retail | 2025 |
| Martini Scaffolding Limited | Construction | 2008 |
| Martins Advisory Ltd | Professional & scientific | 2025 |
| Martins Walk (Borehamwood) Management Limited | Households | 1987 |
| Martlet View (Folders Lane) Management Company Limited | Households | 2023 |
| Martyr Court Limited | Households | 1988 |
| Marugati Ltd | Information & communication | 2019 |
| Marv Limited | Real estate | 2017 |
| Marvelsland Limited | Health & social care | 2022 |
| Mary ANN Workshops Ltd | Households | 2020 |
| Mary Jane Vapes Ltd | Wholesale & retail | 2023 |
| Maryam Prestige Ltd | Wholesale & retail | 2024 |
| MAS Aero Ltd | Transport & logistics | 2017 |
| MAS Connections Ltd | Administrative & support | 2018 |
| MAS Funktion Consulting | Information & communication | 2000 |
| Masala Masters Limited | Hospitality | 2023 |
| Mash Foods Ltd | Hospitality | 2025 |
| Mash Solutions Limited | Real estate | 2019 |
| Masiba Ltd | Wholesale & retail | 2009 |
| Masonry Construction Ltd | Construction | 2021 |
| Masquerade Metaphysicals Ltd | Wholesale & retail | 2026 |
| Massingberd Way Freehold Limited | Households | 2011 |
| Mast Interiors Limited | Construction | 2013 |
| Mastermind Italy Ltd | Wholesale & retail | 2016 |
| Masters in Stone Limited | Manufacturing | 2012 |
| Masters Therapy Limited | Health & social care | 2025 |
| Masterstep Flooring Ltd | Construction | 2019 |
| Mastertechs Diagnostics Limited | Wholesale & retail | 2004 |
| Masum Day Care Services Ltd | Health & social care | 2025 |
| Mataxis Ltd | Professional & scientific | 2010 |
| Matchali Limited | Wholesale & retail | 2025 |
| Matek Business Media Limited | Information & communication | 1994 |
| Mater Arca Limited | Real estate | 2021 |
| Mateusz Caban Ltd | Financial services | 2022 |
| Mather & Platt Limited | Professional & scientific | 1899 |
| Mathi Builders Ltd | Construction | 2025 |
| Mati GSM Ltd | Professional & scientific | 2016 |
| Matico Dance Studio Limited | Arts & recreation | 2023 |
| Matko Limited | Wholesale & retail | 1997 |
| Matodere Solutions Limited | Information & communication | 2025 |
| Matooke Supermarket Ltd | Wholesale & retail | 2020 |
| Matrix Trade London Ltd | Wholesale & retail | 2020 |
| Matsin Care Services Ltd | Health & social care | 2012 |
| Matt Llewellyn-Jones Consulting Ltd | Professional & scientific | 2024 |
| Matt's Property Services Ltd | Real estate | 2026 |
| Mattdewise Limited | Health & social care | 2024 |
| Matthew Cattell Surveyor Ltd | Professional & scientific | 2021 |
| Matthew Thomson Limited | Information & communication | 2015 |
| Matthew's Court Residents Limited | Households | 1987 |
| Maudphy Healthcare Limited | Health & social care | 2019 |
| Maurico (Coffee Run) Ltd | Hospitality | 2023 |
| Maurico Islington Ltd | Hospitality | 2022 |
| Maurico Letchworth Ltd | Hospitality | 2023 |
| Maurico Limited | Wholesale & retail | 2021 |
| Maurico Seven Sisters Ltd | Hospitality | 2022 |
| Maurico ST Neots Ltd | Wholesale & retail | 2024 |
| Maven Property Services Limited | Real estate | 2022 |
| Maven Workforce Ltd | Construction | 2026 |
| Maverick Power Technologies Limited | Administrative & support | 2013 |
| MAX Dixon Property Development Limited | Construction | 2024 |
| MAX Phone & Vape Ltd | Manufacturing | 2025 |
| MAX Power & GAS Limited | Other services | 2025 |
| MAX Power Elite Limited | Health & social care | 2024 |
| MAX Renzland Consultancy Limited | Hospitality | 2012 |
| Maxicam Limited | Wholesale & retail | 2010 |
| Maxima Energy UK Ltd | Energy | 2016 |
| Maxima Telecom Limited | Information & communication | 2012 |
| Maximised Ltd | Health & social care | 2009 |
| Maxwellbiz Limited | Wholesale & retail | 2024 |
| MAY & Stephens Building Solutions Limited | Construction | 2004 |
| MAY & Stephens JOB Search Support Ltd | Administrative & support | 2015 |
| MAY & Stephens Limited | Administrative & support | 1993 |
| MAY Court Management Limited | Households | 2000 |
| MAY Excellence Limited | Health & social care | 2025 |
| Maya Stores Limited | Wholesale & retail | 2025 |
| Maya's Cafe Ltd | Hospitality | 2018 |
| Mayahuel Ltd | Hospitality | 2021 |