| Marchmont Advisory (UK) Limited | Professional & scientific | 2025 |
| Marchwood Engineering Ltd | Manufacturing | 2014 |
| Marci Design Ltd | Construction | 2020 |
| Marciano Holding PC | | 2019 |
| Marcliffproperties Limited | Real estate | 2025 |
| Marco Barber Ltd | Other services | 2023 |
| Marco Hauer | Professional & scientific | 2023 |
| Marco Visconti Ltd | Professional & scientific | 2018 |
| Marco.Hairbusinessconsultancy Ltd | Professional & scientific | 2023 |
| Marcol Asset Management Ltd | Real estate | 2000 |
| Marcol Securities Limited | Other services | 2007 |
| Marcot Property & Finance Limited | Financial services | 2016 |
| Marcus Armitage Golf Ltd | Arts & recreation | 2021 |
| Marcus Armitage Investments Limited | Real estate | 2025 |
| Marcus Brothers Ltd | Information & communication | 2025 |
| Mardan Holdings Systems Limited | Information & communication | 2026 |
| Mardel Landscaping Ltd | Administrative & support | 2025 |
| Mardes Business Services Ltd | Information & communication | 2017 |
| Mardy Investments Limited | Real estate | 2020 |
| Mareece Aesthetics Limited | Other services | 2025 |
| Marel GB Ltd | Wholesale & retail | 1986 |
| Marel Ltd | Manufacturing | 2006 |
| Marell Byrd Limited | Wholesale & retail | 2019 |
| Maren Properties Limited | Real estate | 2017 |
| Mareno Marketing Ltd | Information & communication | 2018 |
| Marexsol | Professional & scientific | 2023 |
| Marfin International Ltd | Information & communication | 2025 |
| Margaret ONI Ltd | Manufacturing | 2012 |
| Marge Motors Ltd | Wholesale & retail | 2017 |
| Margenex Systems Ltd | Information & communication | 2025 |
| Marginal Gains Services Ltd | Real estate | 2008 |
| Margo & MYA Ltd | Professional & scientific | 2018 |
| Maria Farrelly Photography Limited | Professional & scientific | 2013 |
| Maria International UK Ltd | Other services | 2018 |
| Maria Jayne Forster Limited | Health & social care | 2014 |
| Maria Procino Limited | Professional & scientific | 2007 |
| Maria Rahimzada Estates Limited | Real estate | 2024 |
| Maria Smutna Limited | Information & communication | 2026 |
| Marial Energy Consulting Ltd | Mining & quarrying | 2023 |
| Marias Greek Restaurant Ltd | Hospitality | 2022 |
| Marie E Sadler Limited | Education | 2018 |
| Marigold Costumes Limited | Arts & recreation | 2005 |
| Marigold Facilities Limited | Administrative & support | 2026 |
| Marimax Trade PC | | 2014 |
| Marin Associates Limited | Public administration | 2013 |
| Marina Health Works Ltd | Health & social care | 2026 |
| Marine Bespoke Bathrooms and Kitchens Limited | Construction | 2025 |
| Marine Market Ltd | Wholesale & retail | 2020 |
| Marine Parade Properties Ltd | Financial services | 2026 |
| Marine People Limited | Administrative & support | 2017 |
| Marine Post-Fixture Services Limited | Professional & scientific | 2010 |
| Marine Services Global Ltd | Wholesale & retail | 2020 |
| Marine Supply and Logistics PC | | 2018 |
| Marine Tiling Contractors Ltd | Construction | 2023 |
| Mariner Productions Ltd | Information & communication | 2020 |
| Marinex Drilling and Support Services Ltd | Construction | 2019 |
| Maring Precious Ltd | Professional & scientific | 2023 |
| Marino Finance Ltd | Real estate | 2019 |
| Mario Expert Limited | Transport & logistics | 2018 |
| Mario Group Ltd | Extraterritorial | 2017 |
| Mario Installing Ltd | Construction | 2017 |
| Mario Logo Ltd | Information & communication | 2024 |
| Marion Court (Lisvane) Limited | Households | 1982 |
| Mariposa Stay Ltd | Hospitality | 2019 |
| Mariposa Therapeutics Limited | Professional & scientific | 2019 |
| Maris (H) Limited | Financial services | 2019 |
| Maris (UK) Limited | Manufacturing | 1998 |
| Maris Hospitality Limited | Hospitality | 2018 |
| Maris Maldives Developments Ltd | Construction | 2025 |
| Marisum | Professional & scientific | 2025 |
| Marita's Magical Moments Ltd | Wholesale & retail | 2026 |
| Maritime IT Solutions Ltd | Information & communication | 2024 |
| Maritime Register of Shipping (UK) Ltd | Transport & logistics | 1996 |
| Maritime Spices Limited | Wholesale & retail | 2024 |
| Maritime Treasures Ltd | Wholesale & retail | 2025 |
| Mariza Ltd | Wholesale & retail | 2025 |
| Marj Electrical Ltd | Construction | 2026 |
| Mark Andrews Pro Wrestling Limited | Wholesale & retail | 2018 |
| Mark Anthony Properties Limited | Real estate | 2009 |
| Mark Bolwell (Physiotherapists) Limited | Health & social care | 2006 |
| Mark Colbourne MBE Limited | Other services | 2013 |
| Mark Finch Copywriter Limited | Information & communication | 2013 |
| Mark Grant Commercial Business Solutions Ltd | Professional & scientific | 2013 |
| Mark Group Corporate Solutions Limited | Information & communication | 2010 |
| Mark Hoffer Scenery Ltd | Construction | 2011 |
| Mark James Building Contractors Limited | Construction | 2006 |
| Mark James Landscaping Contractors Limited | Construction | 2016 |
| Mark Lewis Investment Properties Ltd | Real estate | 2016 |
| Mark Rand Site Services Limited | Construction | 2017 |
| Mark Rausa Physiotherapy Limited | Other services | 2012 |
| Mark Ridal Courier Services Ltd | Transport & logistics | 2016 |
| Mark Roger Contracts Ltd | Construction | 2021 |
| Mark Sayle Limited | Manufacturing | 2019 |
| Mark Thyme Audio Visual Ltd | Construction | 2013 |
| Mark Williams Consultancy Ltd | Professional & scientific | 2014 |
| Mark Woodrow Properties Ltd | Real estate | 2019 |
| Markdunne Limited | Construction | 2018 |
| Marker Frames Ltd | Professional & scientific | 2026 |
| Market Corporation and Research Limited | Transport & logistics | 2019 |
| Market Hive Ltd | Wholesale & retail | 2024 |