| M.K.Electrix Ltd | Construction | 2014 |
| M1 Goalkeeping Ltd | Education | 2021 |
| M10 Fire Consultancy Limited | Professional & scientific | 2005 |
| M10 Fire Safety Management Limited | Professional & scientific | 2014 |
| M23 Safety Consulting Ltd | Professional & scientific | 2024 |
| M3 Associates Limited | Professional & scientific | 1996 |
| M42 Technologies Limited | Information & communication | 2024 |
| MA & SC Limited | Health & social care | 2020 |
| Ma'At Advisory Ltd | Professional & scientific | 2026 |
| Maab Health Ltd | Professional & scientific | 2023 |
| Maantra Design Limited | Wholesale & retail | 2025 |
| Maas IT Consultancy Ltd | Information & communication | 2008 |
| Mabbott Equestrian Ltd | Wholesale & retail | 2019 |
| Mabteb Property Limited | Real estate | 2016 |
| MAC MAC Ltd | Transport & logistics | 2016 |
| Macfyden Bolt Logistics Ltd | Transport & logistics | 2026 |
| Mach 2020 Ltd | Construction | 2019 |
| Macherts Limited | Real estate | 2009 |
| Machin Enterprises Limited | Administrative & support | 2021 |
| Mack Plus Consultancy Limited | Professional & scientific | 2014 |
| Macmanus Music Ltd | Information & communication | 2022 |
| Macro Thoughts Limited | Professional & scientific | 2017 |
| MAD Socks Marketing Ltd | Information & communication | 2018 |
| Madakaj Ltd | Real estate | 2025 |
| Maddison Television & Media Ltd | Information & communication | 2013 |
| Made Gingerly Limited | Manufacturing | 2023 |
| Madina Halal Meat Centre Ltd | Wholesale & retail | 2025 |
| Madison Avenue Limited | Manufacturing | 2002 |
| Magenta Corporation Limited | Information & communication | 1998 |
| Magenta Recoveries Ltd | Professional & scientific | 2023 |
| Magic Consultancy Ltd | Administrative & support | 2025 |
| Magna Bespoke Ltd | Construction | 2023 |
| Magnolia Gardens (ST Albans) Management Company Limited | Households | 2014 |
| Magnolias Investments Ltd | Real estate | 2019 |
| Magnopus Ltd | Information & communication | 2011 |
| Mahi Platforms Ltd | Information & communication | 2026 |
| Mahiki Limited | Hospitality | 2006 |
| Maimonuk Ltd | Professional & scientific | 2024 |
| Main Project Consultancy Ltd | Professional & scientific | 2020 |
| Main Street Capital Ltd | Wholesale & retail | 2023 |
| Mainline Resourcing Limited | Administrative & support | 2008 |
| Mainsail Property Limited | Construction | 2025 |
| Mair & Woods Ltd | Professional & scientific | 2015 |
| Maison D'Etre Investments Ltd | Real estate | 2012 |
| Maison Hartley Limited | Wholesale & retail | 2012 |
| Maison KIM Ltd | Hospitality | 2019 |
| Maison Sassy UK Limited | Wholesale & retail | 2018 |
| Majestic Group Limited | Wholesale & retail | 2000 |
| Majestic Landscape Products Limited | Wholesale & retail | 2000 |
| Majestic Landscapes Limited | Administrative & support | 2000 |
| Major Hospitality Solutions Limited | Real estate | 2019 |
| Make Headway Ltd | Information & communication | 2020 |
| Make IT Beautiful Theatre Company Ltd | Arts & recreation | 2022 |
| Make IT Work Consulting Limited | Professional & scientific | 2008 |
| Make the Leap Limited | Administrative & support | 2001 |
| Makerlink Limited | Wholesale & retail | 2023 |
| Makin Locations Limited | Arts & recreation | 2018 |
| Making Waves Swimming Limited | Arts & recreation | 2025 |
| Mako Land Limited | Construction | 2020 |
| Maktig Ltd | Information & communication | 2007 |
| Malibu Health Products Limited | Manufacturing | 1999 |
| Malibu Limited | Manufacturing | 2005 |
| Malibu Manufacturing Ltd | Manufacturing | 2013 |
| Malibu Trading Ltd | Other services | 2012 |
| Malibu World Shopping Ltd | Other services | 2012 |
| Malik Investment Management Limited | Real estate | 2018 |
| Mallard Court (Baldock) Management Limited | Administrative & support | 2013 |
| Maltby Corporate Limited | Professional & scientific | 2016 |
| Maltings Communications Limited | Wholesale & retail | 2022 |
| MAMAL86 Limited | Hospitality | 2025 |
| Mammoth Beer Limited | Manufacturing | 2020 |
| Mandalay Foods Ltd | Hospitality | 2019 |
| Mandarin Design LLP | | 2025 |
| Mango Properties Ltd | Real estate | 2000 |
| Manhattan Beaconsfield Limited | Wholesale & retail | 1997 |
| Manifest 8888 Ltd | Wholesale & retail | 2024 |
| Manifesto Group Limited | Information & communication | 2000 |
| Manjunath Limited | Health & social care | 2023 |
| Manlift Manufacturing Limited | Manufacturing | 2021 |
| Manna Music Limited | Arts & recreation | 1990 |
| Manor Courtyard Management Company 2021 Ltd | Households | 2021 |
| Manor Group Consulting Ltd | Professional & scientific | 2024 |
| MANSARDS2 Ltd | Real estate | 2022 |
| Mansion House (Chiswell Green) Limited | Construction | 2026 |
| Mansyl Limited | Information & communication | 2023 |
| Manta Capital Rarecells SPV 1 LLP | | 2023 |
| Manta Capital Rarecells SPV 2 LLP | | 2024 |
| Mantra Lingua Holdings Limited | Financial services | 2021 |
| Mantra Lingua Limited | Information & communication | 2002 |
| Manzin Consulting Ltd | Professional & scientific | 2014 |
| Maple Casements Limited | Construction | 2024 |
| Marc Richard Ltd | Professional & scientific | 2019 |
| Marchium Law Limited | Professional & scientific | 2024 |
| Marford Barns Management Company Ltd | Households | 2020 |
| Margrock Limited | Real estate | 2023 |
| Margs Media Ltd | Professional & scientific | 2024 |
| Maria E Kariadi Limited | Information & communication | 2019 |
| Marina Property Investments Ltd | Real estate | 2025 |
| Marine Components International Limited | Transport & logistics | 1990 |
| Mariposa Consulting Limited | Information & communication | 2022 |