| Marlek Limited | Manufacturing | 2011 |
| Marnico Underwriting Limited | Financial services | 2009 |
| Marris Farming Ltd | Agriculture & fishing | 2024 |
| Marris Foston Limited | Agriculture & fishing | 2005 |
| Marshribeam Ltd | Administrative & support | 2024 |
| Martin EBB Stonemasonry Limited | Manufacturing | 2023 |
| Martin Wand Ltd | Other services | 2026 |
| Martini Media Management Ltd | Professional & scientific | 2021 |
| Mary P Holdings Ltd | Financial services | 2023 |
| MAS Medical Consultants Limited | Health & social care | 2016 |
| Masala Junction Grantham Ltd | Hospitality | 2025 |
| Maschavison Ltd | Administrative & support | 2021 |
| Masons Paddock Management Company Limited | Households | 2018 |
| Masscomedia UK Ltd | Wholesale & retail | 2015 |
| Mathew Neville Limited | Professional & scientific | 2026 |
| Matt F Construction Ltd | Construction | 2019 |
| Matt Finished Limited | Construction | 2012 |
| MAW Business Ltd | Professional & scientific | 2010 |
| Maxfield Property Developments Limited | Real estate | 2007 |
| Maxi Paint Ltd | Wholesale & retail | 2023 |
| Mayfleet Recruitment Limited | Information & communication | 2024 |
| MB Secure Limited | Other services | 2017 |
| MBN Commercial Properties Ltd | Real estate | 2026 |
| Mbst Health Limited | Wholesale & retail | 2025 |
| Mccorquodale Estates Stoke Rochford Ltd | Financial services | 2024 |
| Mclean Electrical Systems Ltd | Construction | 2012 |
| MCW Solutions Ltd | Construction | 2024 |
| MD Jewellers Ltd | Manufacturing | 2018 |
| MDC Trading & Loans Limited | Financial services | 2022 |
| MDF Plant Services Limited | Manufacturing | 2009 |
| MDR Building Ltd | Construction | 2017 |
| Meacock 727 Limited | Financial services | 2013 |
| Meadow Lodges Boothby Ltd | Hospitality | 2023 |
| Meadowlime Ltd | Real estate | 1999 |
| Measured Energy Ltd | Extraterritorial | 2016 |
| MEC Media Ltd | Professional & scientific | 2026 |
| Medoc Wines Limited | Wholesale & retail | 2006 |
| Mega Van Mats Ltd | Wholesale & retail | 2025 |
| Mehar FIC Limited | Real estate | 2025 |
| Meili Barber Ltd | Other services | 2022 |
| MELEK18 Catering Limited | Hospitality | 2025 |
| Melessett Ltd | Manufacturing | 2024 |
| Meli Trading Ltd | Wholesale & retail | 2019 |
| Mellions Limited | Professional & scientific | 2026 |
| Memorial Perfection Limited | Manufacturing | 2021 |
| Mente AI Ltd | Other services | 2024 |
| Mercia Innovations Ltd | Professional & scientific | 2015 |
| MF Electrical - Renewable Energy Solutions Ltd | Construction | 2023 |
| MFD Overhead Line Services Limited | Manufacturing | 2014 |
| MFO Life Sciences Ltd | Manufacturing | 2025 |
| MG Greenheat Renewables Limited | Construction | 2020 |
| MG Plastering & Coatings Limited | Construction | 2014 |
| MGJ Associates Ltd | Professional & scientific | 2024 |
| Micari Engineering Solutions Limited | Construction | 2006 |
| Michael Cowen Underwriting LLP | | 2009 |
| Michael Sharp & Co Limited | Real estate | 1959 |
| Michaelmas Trading LLP | | 2009 |
| Michal Gajewski Trading Ltd | Financial services | 2026 |
| Microcapture Ltd | Manufacturing | 2007 |
| Middle House (Askrigg) Limited | Hospitality | 2023 |
| Middleton Aesthetics Limited | Other services | 2018 |
| Midlands Children’S Physio Ltd | Health & social care | 2021 |
| Midlands Property Ltd | Real estate | 2025 |
| Miee Limited | Wholesale & retail | 2023 |
| Mihill Consulting Limited | Administrative & support | 2003 |
| Milestone Consultants Ltd | Professional & scientific | 2025 |
| Miley Squared Consulting Limited | Information & communication | 2014 |
| Milfield Holdings Ltd | Real estate | 2002 |
| Mill Farm Stud Limited | Agriculture & fishing | 2013 |
| Mill Hill Capital LLP | | 2006 |
| Mill Hill Developments Limited | Real estate | 1963 |
| Mill Lodge Cattery Ltd | Agriculture & fishing | 2003 |
| Millhouse Renovations Limited | Construction | 2009 |
| Minima Rome Limited | Financial services | 2011 |
| Minna Blooms Ltd | Extraterritorial | 2021 |
| Mint Live Group Ltd | Administrative & support | 2020 |
| Miraflor Santiago Limited | Health & social care | 2022 |
| Miss Leading Events Ltd | Administrative & support | 2022 |
| Missed Opportunity Media Ltd | Information & communication | 2024 |
| Mitho Chicken Ltd | Hospitality | 2022 |
| MJ One Real Estate Limited | Real estate | 2022 |
| MJC Consultants (UK) Ltd | Financial services | 2010 |
| MJG Thermal Systems Ltd | Construction | 2025 |
| MJS Developments (Lincs) Limited | Hospitality | 2021 |
| MK Melina Ltd | Transport & logistics | 2026 |
| MK Wheel Refinishing Limited | Wholesale & retail | 2020 |
| MKP Brickwork Ltd | Construction | 2025 |
| MLH Electrical Solutions Ltd | Construction | 2023 |
| MM Rutland Ltd | Other services | 2015 |
| MM W&W Management Ltd | Professional & scientific | 2022 |
| MMP 28 Ltd | Hospitality | 2023 |
| Mobi-Gp Ltd | Health & social care | 2024 |
| Mobiking Grantham Limited | Wholesale & retail | 2022 |
| Mobisingh Limited | Wholesale & retail | 2025 |
| Mogensen Sizers Limited | Professional & scientific | 1967 |
| Momenti Group Limited | Professional & scientific | 2025 |
| Money Options Group Limited | Financial services | 2008 |
| Money Scan Ltd | Wholesale & retail | 2011 |
| Monksfield LLP | | 2007 |
| Monopsony Limited | Professional & scientific | 2004 |