| Machen After School Club | Health & social care | 2007 |
| Machen RFC & Community Club CIC | Hospitality | 2025 |
| Machen RFC Limited | Arts & recreation | 2019 |
| Macmanus Asset Finance Ltd | Financial services | 2006 |
| Macmanus Investments Ltd | Real estate | 2020 |
| MAD Hockey CIC | Education | 2025 |
| Maid 4 Work Ltd | Administrative & support | 2022 |
| Mailboxcsbs Limited | Transport & logistics | 2018 |
| Make 1 Ltd | Manufacturing | 2017 |
| Malin Align Scaffolding Limited | Construction | 2024 |
| Mallie Developments Ltd | Real estate | 2024 |
| Manna Education CIC | Education | 2024 |
| Maple Consulting Ltd | Professional & scientific | 2015 |
| Marc Nash Plumbing & Heating Ltd | Construction | 2023 |
| Mariusz Transport Ltd | Extraterritorial | 2021 |
| Mark Brown Property Services Limited | Construction | 2017 |
| Marka Enterprises Limited | Wholesale & retail | 2006 |
| Markat Holdings Limited | Financial services | 2025 |
| Market Curry Ltd | Hospitality | 2020 |
| Marketing Collab Limited | Professional & scientific | 2025 |
| Marketing TOM Media Ltd | Information & communication | 2007 |
| Marlo House Ltd | Hospitality | 2021 |
| Marnell Medical Services Ltd | Health & social care | 2011 |
| Marshmallow Events Limited | Arts & recreation | 2024 |
| Masbe IT Ltd | Information & communication | 2024 |
| Masconic Limited | Wholesale & retail | 2025 |
| Masons Arms (Caerphilly) Ltd | Hospitality | 2022 |
| Matest UK Limited | Wholesale & retail | 2022 |
| Matthew Britton Illustration Limited | Information & communication | 2015 |
| Matthew Bushen Digital Ltd | Information & communication | 2022 |
| Maverick Pro Limited | Extraterritorial | 2024 |
| MAX Couriers Ltd | Transport & logistics | 2014 |
| Maxima Protection Ltd | Administrative & support | 2018 |
| Maxteck PC | | 2018 |
| Mayflower Central Limited | Hospitality | 2023 |
| Mayflower Hygiene Supplies (London) Limited | Wholesale & retail | 2001 |
| MBS Holdings Ltd | Administrative & support | 2023 |
| MC Drylining Limited | Construction | 2021 |
| MC Saray Ltd | Hospitality | 2015 |
| MC2MB Ltd | Hospitality | 2023 |
| Mccarthy Enterprises Ltd | Extraterritorial | 2018 |
| Mccarthy Lettings Limited | Real estate | 2019 |
| Mccarthy Talent Partners Ltd | Education | 2025 |
| Mccarthy Wealth Planning Limited | Financial services | 2020 |
| Mcgrouse Limited | Hospitality | 2016 |
| Mckenzie Consulting Limited | Professional & scientific | 2017 |
| Mctp Trading Ltd | Financial services | 2023 |
| MDD Fabrication & Engineering Limited | Manufacturing | 2022 |
| MDK Trade Ltd | Hospitality | 2025 |
| Meadpropertyteam Ltd | Real estate | 2023 |
| Mears Consulting Limited | Professional & scientific | 2025 |
| Mechtech Professionals Employee Ownership Trustee Limited | Professional & scientific | 2024 |
| Mechtech Professionals Ltd | Administrative & support | 2013 |
| Medsu Ltd | Health & social care | 2012 |
| Megaennium Ltd | Information & communication | 2024 |
| Megan Morgan Limited | Professional & scientific | 2024 |
| Megmillgurk Ltd | Wholesale & retail | 2026 |
| MEL John Properties Limited | Real estate | 2025 |
| Melyn Construction Limited | Construction | 2019 |
| Melyn Facilities Limited | Construction | 2025 |
| Mentor Property Portfolio Ltd | Real estate | 2018 |
| Merched Gwyllt Ltd | Wholesale & retail | 2025 |
| Mercury Manufacturing Ltd | Manufacturing | 2021 |
| Merk Holdings Ltd | Real estate | 2023 |
| Merthyr Lettings Limited | Real estate | 2024 |
| Metis Mortgages & Protection Ltd | Administrative & support | 2022 |
| MGC Renewable Energy Solutions Limited | Construction | 2022 |
| MIA Bella Boutique Limited | Other services | 2019 |
| Michael Hobbs Landscape Gardens and Design Limited | Professional & scientific | 2020 |
| Michelle’S Laundry Box Ltd | Administrative & support | 2020 |
| Midat Ltd | Construction | 2021 |
| Middleton Global Enterprises Limited | Financial services | 2024 |
| Midland Motor Group Ltd | Wholesale & retail | 2020 |
| Mifibre Limited | Information & communication | 2017 |
| Mighty Oaks Tree Solutions Ltd | Agriculture & fishing | 2022 |
| MII Engineering Limited | Professional & scientific | 2009 |
| MII Total Access Limited | Construction | 2016 |
| MII Trustee Limited | Professional & scientific | 2025 |
| Mike Griffiths Ltd | Financial services | 2006 |
| Mikes MOT Limited | Wholesale & retail | 2026 |
| Mill Road Local Limited | Wholesale & retail | 2025 |
| Millennium Fencing Limited | Manufacturing | 2006 |
| Mills Competitions Ltd | Wholesale & retail | 2025 |
| Millzonwheels Ltd | Transport & logistics | 2025 |
| Milton Technical Services Ltd | Manufacturing | 2023 |
| Mirage Blinds Limited | Manufacturing | 1993 |
| Misfit Bars Ltd | Hospitality | 2024 |
| Misfit Brewing Company Ltd | Manufacturing | 2022 |
| Miss Victoria School of Ballet Ltd. | Education | 2021 |
| Mitchell's Productions Limited | Arts & recreation | 2023 |
| MJ Management Services Ltd | Construction | 2018 |
| MJB Digital Consulting Ltd | Information & communication | 2017 |
| MJK Transaction Limited | Manufacturing | 2012 |
| MJL Property and Garden Services Ltd | Households | 2025 |
| MK Equine and Pet Sitting Services Ltd | Agriculture & fishing | 2025 |
| MK Properties Development Ltd | Real estate | 2024 |
| MM Accountancy Services Ltd | Professional & scientific | 2025 |
| Mobil Information Systems UK Ltd | Information & communication | 2006 |
| Mogstone Ltd | Administrative & support | 2018 |
| Mohayes Property Services Ltd | Construction | 2020 |