| MAS Mechanical Limited | Construction | 2006 |
| Masala Marvels Ltd | Hospitality | 2024 |
| Masbra Limited | Administrative & support | 2010 |
| Maso Limited | Manufacturing | 2026 |
| Mason Contract Services Limited | Construction | 2019 |
| Massey Windows & Doors Limited | Construction | 2020 |
| Massive ADS Digital Limited | Professional & scientific | 2016 |
| Mastaly Limited | Construction | 2019 |
| Master Clean 24/7 Ltd | Administrative & support | 2022 |
| Master Consulting Ltd | Education | 2018 |
| Master Digital Services Limited | Transport & logistics | 2011 |
| Master Exteriors Ltd | Construction | 2022 |
| Masterclass Removals Limited | Transport & logistics | 2015 |
| Mastering UI Ltd | Information & communication | 2008 |
| Masukh Ltd | Wholesale & retail | 2023 |
| Matcen Limited | Health & social care | 2015 |
| Matei Engineering and Surveying Services Ltd | Administrative & support | 2021 |
| Matei RM Ltd | Transport & logistics | 2018 |
| Mateusz Madrzyk Ltd | Construction | 2018 |
| Mate’Balls Ltd | Hospitality | 2026 |
| Mathetha Limited | Health & social care | 2015 |
| Mathews Consultants Ltd | Professional & scientific | 2025 |
| Matrix Fire Alarms Ltd | Public administration | 2011 |
| Mats Maintenance Thurrock Ltd | Manufacturing | 2024 |
| Matt Steel Limited | Professional & scientific | 2013 |
| Matter of Time Ltd | Information & communication | 2024 |
| Matteuscaribbeancuisine Ltd | Hospitality | 2023 |
| Matthewman's Medical Services Limited | Health & social care | 2023 |
| Mavadia Ventures Limited | Extraterritorial | 2013 |
| Mavelle (UK) Ltd | Transport & logistics | 2014 |
| MAX Enterprises London Limited | Administrative & support | 2020 |
| MAX Epsilon Ltd | Wholesale & retail | 2026 |
| MAX Witting Ltd | Information & communication | 2016 |
| MAX4CLEAN Limited | Administrative & support | 2013 |
| Maxb Ltd | Construction | 2012 |
| Maxd Trans Ltd | Transport & logistics | 2022 |
| Maxilite Ltd | Manufacturing | 2018 |
| Maxwell Dynamics Limited | Information & communication | 2025 |
| Maxx Fixing Limited | Construction | 2016 |
| Maxz Ltd | Transport & logistics | 2016 |
| MAY Fine Art Ltd | Extraterritorial | 2020 |
| MAY Solutions UK Limited | Administrative & support | 2014 |
| May's Spot 13 Limited | Hospitality | 2026 |
| Mayari Food Limited | Wholesale & retail | 2025 |
| Mayer Developments Limited | Construction | 2022 |
| Mayfield Property Company (Vange) Limited | Real estate | 2016 |
| Mayfly Wine Company Ltd. | Wholesale & retail | 2016 |
| Mayland Thermal Limited | Construction | 2014 |
| Maylife Healthcare Limited | Health & social care | 2025 |
| Maytree Properties Limited | Construction | 1995 |
| May’S Bakery and Patisserie Ltd | Hospitality | 2021 |
| Mazie Ltd | Construction | 2015 |
| Mazio Ltd | Wholesale & retail | 2025 |
| MB Cladworks Ltd | Construction | 2026 |
| MB Painters Essex Ltd | Construction | 2023 |
| Mb.Seven Steelwork Ltd | Construction | 2026 |
| MBC Delivery Ltd | Construction | 2024 |
| MBC Plumbing Limited | Construction | 2019 |
| MBF Utilities Limited | Other services | 2015 |
| MBM Construction & Highways Maintenance Limited | Construction | 2018 |
| MBM Gardening & Tree Care Ltd | Administrative & support | 2025 |
| MBN Solutions Ltd | Wholesale & retail | 2020 |
| MC Company Private Ltd | Wholesale & retail | 2025 |
| MC Draughting Services Limited | Administrative & support | 1997 |
| MC Perfect Services Ltd | Construction | 2016 |
| MC Property Solutions Essex Limited | Construction | 2023 |
| Mc.Ana Ltd | Construction | 2026 |
| Mcaron Care Service Ltd | Health & social care | 2022 |
| Mccabe I.T. Ltd | Information & communication | 2026 |
| Mccabe Marketing Ltd | Professional & scientific | 2026 |
| Mccrimmon Publishing Company Limited | Information & communication | 1969 |
| Mcgowan Motor Company Ltd | Wholesale & retail | 2021 |
| Mcgowan Plant & Excavation Limited | Administrative & support | 2018 |
| Mcguinness Homes Limited | Real estate | 2021 |
| Mcintee Limited | Hospitality | 2025 |
| Mcir Ltd | Other services | 2015 |
| MCM Fire and Security Limited | Administrative & support | 2023 |
| Mcquilliam Property Limited | Real estate | 2024 |
| MD Environmental Ltd | Water & waste | 2024 |
| MD Logistics (Luton) Limited | Construction | 2021 |
| MD Real Estate London Ltd | Real estate | 2025 |
| Md&Ma Ltd | Construction | 2025 |
| Md-East Limited | Administrative & support | 2024 |
| MD86 Holding Limited | Financial services | 2026 |
| MD86 Properties Limited | Real estate | 2026 |
| MDA Projects Ltd | Construction | 2016 |
| MDA Property Solutions Ltd | Real estate | 2025 |
| MDB Injury & Rehabilitation Clinic Ltd | Health & social care | 2019 |
| Mdluli Care Services Ltd | Health & social care | 2015 |
| MDM Timber Limited | Wholesale & retail | 1989 |
| MDR Properties Essex Limited | Real estate | 2024 |
| MDS Contracting Group Limited | Extraterritorial | 2013 |
| MDS Contracts Limited | Construction | 2015 |
| MDW Automotive Ltd | Wholesale & retail | 2025 |
| MDW Investments Ltd | Real estate | 2021 |
| ME Old China Catering Limited | Hospitality | 2019 |
| Meadmark Limited | Construction | 1979 |
| Mebax Ltd | Construction | 2025 |
| Mecca UK Limited | Other services | 2015 |
| Mechanical Air Services Ltd | Construction | 2014 |