| Macdowall Equipment Company Limited | Construction | 1939 |
| Mack Haulage Ltd | Transport & logistics | 2015 |
| Madisons Fire Ltd | Manufacturing | 2024 |
| Magyar Thistle Ltd | Administrative & support | 2022 |
| Main Property Limited | Real estate | 2018 |
| Mainereli Ltd | Real estate | 2026 |
| Mainframe Contracts Ltd | Manufacturing | 2021 |
| Mainframe Fabrications Ltd | Manufacturing | 2016 |
| Maison DE Florence Ltd | Wholesale & retail | 2021 |
| MAK Energy Ltd | Construction | 2008 |
| Make Wickford Great Again Ltd | Hospitality | 2024 |
| Maldon Swimskool Limited | Education | 2011 |
| Mallard and Bittern Court Management Company Limited | Households | 1975 |
| Malting Villas Property Ltd | Real estate | 2019 |
| Malyon Investments Limited | Real estate | 2017 |
| Manaca' Estate Limited | Administrative & support | 2013 |
| Managencies Limited | Professional & scientific | 2023 |
| Managers of Wealth Limited | Administrative & support | 2026 |
| Managing Construction Projects Ltd | Construction | 2012 |
| Manikam Wealth Management Limited | Professional & scientific | 2010 |
| Mansfield Scaffold Merchants Limited | Wholesale & retail | 2016 |
| Mansom Limited | Wholesale & retail | 2010 |
| Manufacturers and Suppliers of Lighting Limited | Wholesale & retail | 2023 |
| Maple Builders Ltd | Construction | 2002 |
| Maple Carpentry & Joinery Limited | Manufacturing | 2014 |
| Maptec Plumbing and Heating Ltd | Construction | 2021 |
| Mapuk (Holdings) Limited | Financial services | 2020 |
| Mapuk Developments LLP | | 2020 |
| Mara Carpentry & Joinery Limited | Manufacturing | 2026 |
| Marbel HOMES83 Limited | Real estate | 2019 |
| Marble & Granite Works Limited | Manufacturing | 2004 |
| Marc's Tyres & Autocare Ltd | Professional & scientific | 2022 |
| Marcopolo Food Ltd | Hospitality | 2022 |
| Margerum's Limited | Wholesale & retail | 1955 |
| Marina Boat Covers Limited | Manufacturing | 2018 |
| Mark Cooper Decorating Ltd | Construction | 2011 |
| Mark Janney Limited | Other services | 2019 |
| Mark's Discount Stores Limited | Real estate | 1971 |
| Marlow Developments & Building Limited | Construction | 2015 |
| Marlowe Wealth Management Ltd | Financial services | 2015 |
| Marsel Decoration Ltd | Construction | 2017 |
| Marshal Digital Limited | Wholesale & retail | 2025 |
| Marshalls of Conwy Ltd | Manufacturing | 2021 |
| Martin Higgs Media Ltd | Professional & scientific | 2022 |
| Mascot Ventures Ltd | Information & communication | 2022 |
| Match Point Events Ltd | Arts & recreation | 2024 |
| Maths 360 Ltd | Education | 2018 |
| Matrix Catering Systems Ltd. | Manufacturing | 1994 |
| Matthew Electrical & Surveying Limited | Construction | 2023 |
| Matthews Electrical Installations Limited | Construction | 2021 |
| Matthews Rail Infrastructure Services Limited | Construction | 2016 |
| Mattie Limited | Health & social care | 2023 |
| Mattlore Ltd | Wholesale & retail | 2025 |
| Mavrastar Limited | Real estate | 1983 |
| MAX Ersser Technical Limited | Construction | 2025 |
| Mayfair Trunk Company Limited | Wholesale & retail | 2004 |
| Mazena Limited | Hospitality | 2024 |
| Mazlo Limited | Hospitality | 2019 |
| MB Carpets and Flooring Ltd | Construction | 2015 |
| MBT Investment Holdings Limited | Real estate | 2025 |
| MCM Finishing Limited | Manufacturing | 2014 |
| MCM Joinery Solutions Limited | Manufacturing | 2002 |
| Mcstone Investments Limited | Real estate | 2022 |
| MCW Business & Legal Solutions Ltd | Professional & scientific | 2023 |
| MD Care Ltd | Administrative & support | 2013 |
| MD Property Limited | Real estate | 2019 |
| MDD Woodchip Limited | Wholesale & retail | 2019 |
| MDJ Partnered Solutions Ltd | Professional & scientific | 2018 |
| MDP Systems Limited | Information & communication | 2009 |
| Mdrw Consultancy Ltd | Professional & scientific | 2018 |
| ME VS. ME Studio Ltd | Wholesale & retail | 2025 |
| Media and Events Recruitment Limited | Administrative & support | 2021 |
| Medical DR Ltd | Health & social care | 2014 |
| Medical Health Solutions Ltd | Health & social care | 2021 |
| Meditech Investments Limited | Administrative & support | 2014 |
| Medley Arts Ltd | Arts & recreation | 2025 |
| Medori Consulting Ltd | Professional & scientific | 2026 |
| Medway Flooring Centre Limited | Wholesale & retail | 2006 |
| Medway Flooring Contracts Ltd | Extraterritorial | 2020 |
| Medway Flooring Specialists Ltd | Extraterritorial | 2020 |
| Meebles Limited | Professional & scientific | 1958 |
| Megacorp Accountants Limited | Professional & scientific | 2001 |
| Megacorp Properties Limited | Real estate | 2016 |
| Megz Kingston Coaching Ltd | Education | 2023 |
| Meilleur Consulting Ltd | Extraterritorial | 2022 |
| Melfera Ltd | Real estate | 2020 |
| Melim Financial Services Limited | Professional & scientific | 2019 |
| Melrose Engineering Consultants Ltd | Professional & scientific | 2019 |
| Memikoglu Property Ltd | Real estate | 2025 |
| Memorial Consultancy Services Ltd | Manufacturing | 2018 |
| Men's Fertility Limited | Health & social care | 2023 |
| Menever Construction Limited | Professional & scientific | 2022 |
| Merchant Hospitality (Essex) Ltd | Hospitality | 2025 |
| Mercury Models Limited | Manufacturing | 1955 |
| Mercury Property Investments Ltd | Construction | 2023 |
| Mercury Protection Services Limited | Administrative & support | 2018 |
| Meta Dating Limited | Other services | 2021 |
| Metchem Marine Logistics Ltd | Professional & scientific | 2017 |
| Metchem Marine Solutions Ltd | Wholesale & retail | 2020 |
| Meteor Design Ltd | Professional & scientific | 2022 |