| Mar-Key Group Limited | Manufacturing | 1991 |
| Marc Waple Limited | Professional & scientific | 2025 |
| Marella Residents Management Co Ltd | Households | 2024 |
| Marina View Tuckton Management Co. Limited | Households | 1987 |
| Marine AID Limited | Manufacturing | 2003 |
| Marine Audio Visual Ltd | Wholesale & retail | 2022 |
| Marine Electrification Solutions Ltd | Construction | 2018 |
| Mark Brown Commercial Property Services Limited | Real estate | 2019 |
| Mark Hammill Limited | Information & communication | 2015 |
| Mark Rutter Limited | Health & social care | 2012 |
| Mark Seal Ltd | Wholesale & retail | 2012 |
| Marketing Data Options Ltd | Other services | 2010 |
| Markham Financial Services Ltd | Financial services | 2009 |
| Markwar Limited | Construction | 2006 |
| Marlborough Place Management Company Limited | Households | 1995 |
| Marybourne Limited | Households | 2015 |
| Marylove Ltd | Real estate | 2015 |
| Masegolefa Ltd | Health & social care | 2022 |
| Masku Limited | Wholesale & retail | 2015 |
| Matchbox Fishing Tackle Ltd | Wholesale & retail | 2018 |
| Mati Delicatessen Ltd | Wholesale & retail | 2024 |
| Matisse Beauty Clinic Limited | Other services | 2013 |
| Matopo Property Company Limited | Financial services | 2014 |
| Matsu Trading Ltd | Professional & scientific | 2022 |
| Maui Creative Ltd | Professional & scientific | 2022 |
| MAV Electrical Services Ltd | Construction | 2025 |
| Mawford Ltd | Hospitality | 2021 |
| MAY Eight Holdings Limited | Professional & scientific | 2022 |
| Mayaandmoon Limited | Arts & recreation | 2025 |
| Mayes Windows (Christchurch) Limited | Construction | 2009 |
| Mayflower Atlantic Limited | Information & communication | 2025 |
| Maze IT Limited | Information & communication | 2016 |
| Mccarthy Solicitors Ltd | Professional & scientific | 2022 |
| Mcdevitt Roofing Services Ltd | Construction | 2024 |
| Mcgrane Optical Limited | Health & social care | 2022 |
| Mcnulty Solutions Ltd | Professional & scientific | 2026 |
| MCW Window Cleaning Ltd | Administrative & support | 2025 |
| MD Johnson Electrical (South) Limited | Construction | 2022 |
| MDV Stores Limited | Wholesale & retail | 2020 |
| ME Newforest Ltd | Health & social care | 2022 |
| Meadowbank Holidays Limited | Hospitality | 1975 |
| Meadowbrook Catering Ltd | Wholesale & retail | 2010 |
| Meadowland Construction Ltd | Real estate | 2024 |
| Meakes Scenery Ltd | Construction | 2023 |
| Media Trails Ltd | Information & communication | 2021 |
| Medina Consultancy Limited | Professional & scientific | 2025 |
| Medvale Nutrition Limited | Wholesale & retail | 2025 |
| Medway Valley Fisheries Limited | Arts & recreation | 2006 |
| Mega Mayhem Limited | Arts & recreation | 2026 |
| Memories NOT Dreams Holding Company Ltd | Financial services | 2024 |
| Mental Capacity Solutions Limited | Health & social care | 2023 |
| Merlin's Catering Limited | Hospitality | 2003 |
| Merton Court(Highcliffe-On-Sea)Limited | Households | 1959 |
| Metalbox Storage Ltd | Transport & logistics | 2016 |
| Metulas Supplements Ltd | Health & social care | 2022 |
| Meyrick Estate Solar Management Ltd | Energy | 2022 |
| Meyrick Group Limited | Financial services | 2023 |
| MFG (NO. 2) Limited | Extraterritorial | 2012 |
| Mfgp Holdings Limited | Financial services | 2016 |
| MG Carpentry & Build Ltd | Construction | 2021 |
| MG Randalls Way Limited | Construction | 2016 |
| Mi.Seven Limited | Transport & logistics | 2020 |
| Micren Limited | Financial services | 1994 |
| Mighty Oaks Day Nursery & Preschool Limited | Education | 2017 |
| Milborne Ltd | Construction | 2015 |
| Mildren Construction EOT Limited | Administrative & support | 2023 |
| Mildren Construction Limited | Construction | 1987 |
| Mildren Homes Central Limited | Construction | 2015 |
| Miles Farming UK Limited | | 2022 |
| Miles Tooley Limited | Financial services | 2017 |
| Millstream Partners Ltd | Professional & scientific | 2020 |
| Milner Management Consultancy Ltd | Other services | 2022 |
| Minco Design and Print Limited | Professional & scientific | 2012 |
| Mindful Echo Ltd | Information & communication | 2026 |
| Miraculous Stitches Ltd | Arts & recreation | 2019 |
| Missionx Service Company Ltd | Professional & scientific | 2022 |
| MJC Innovation Ltd | Manufacturing | 2025 |
| MJM Scaffolding Ltd | Construction | 2007 |
| MK Bidco Limited | Financial services | 2023 |
| MK Decor Ltd | Construction | 2021 |
| MK Topco Limited | Financial services | 2023 |
| ML Automation Limited | Information & communication | 2013 |
| MME Holdings Limited | Manufacturing | 2015 |
| Mmedia Group Limited | Professional & scientific | 2004 |
| Mmedia Holdings Limited | Financial services | 2013 |
| Mobile Consultancy Solutions Limited | Extraterritorial | 2010 |
| Mobile Media Properties Limited | Real estate | 2015 |
| Modulus Seating Limited | Manufacturing | 2009 |
| Mojo Chiropractic Ltd | Health & social care | 2022 |
| Mojo People Consulting Ltd | Professional & scientific | 2006 |
| Molly's Shopping Villages Limited | Wholesale & retail | 2017 |
| Momentum Pages Ltd | Information & communication | 2026 |
| Monitus Medical Limited | Professional & scientific | 2019 |
| Monkey 33 Limited | Wholesale & retail | 2020 |
| Montagu Projects Ltd | Construction | 2024 |
| Montagu Road Management (Highcliffe) Ltd | Hospitality | 2015 |
| Moore Information Systems Limited | Information & communication | 2002 |
| Moorings Tuckton Management Co. Limited(The) | Households | 1986 |
| Moorlands College | Education | 2001 |
| More Executive Ltd | Professional & scientific | 2014 |