| Marc Antoni (Fleet) Limited | Other services | 2013 |
| Marc Antoni (Henley) Limited | Other services | 2013 |
| Marc Antoni (UK) Limited | Financial services | 2006 |
| Marc Antoni (Woodley) Limited | Other services | 2013 |
| Marc Antoni Limited | Extraterritorial | 2014 |
| March Hare Holdings Ltd | Financial services | 2023 |
| Margaretta Lane Ltd | Real estate | 2023 |
| Margin Properties Ltd | Real estate | 2025 |
| Marine Pre-Packs Limited | Wholesale & retail | 2011 |
| Mark Alsop Associates Limited | Professional & scientific | 2017 |
| Mark Birchall Limited | Information & communication | 2017 |
| Mark Elsdon Game Design Ltd | Professional & scientific | 2026 |
| Mark Innes Plumbing & Heating Limited | Construction | 2016 |
| Mark James Flooring Ltd | Construction | 2015 |
| Marked by Miss Ltd | Education | 2024 |
| Marlin PUB Co Ltd | Hospitality | 2015 |
| Marmi Bespoke Stone Interiors Limited | Other services | 1997 |
| Marmi Ltd. | Construction | 1992 |
| Marshmallow Cosmetics Ltd | Administrative & support | 2023 |
| Martin Penny Limited | Real estate | 1990 |
| Marvellous Munch Limited | Hospitality | 2017 |
| Masters Business Protection Ltd | Administrative & support | 2022 |
| Material Processing Solutions Limited | Professional & scientific | 2019 |
| Maternity Cover Ltd | Administrative & support | 2018 |
| Matt Bacon Development Ltd | Information & communication | 2022 |
| Matt Ford Tennis Ltd | Education | 2023 |
| Matter Designs Interiors Limited | Wholesale & retail | 2024 |
| Matthew Stiff Limited | Professional & scientific | 2021 |
| MAY & Butcher Ltd | Other services | 2021 |
| May.De Design Limited | Professional & scientific | 2019 |
| Mayday Accountants Ltd | Professional & scientific | 2013 |
| Mayflower Shipping Limited | Transport & logistics | 1991 |
| Mayland Storage & Manufacturing Limited | Professional & scientific | 2005 |
| Mayland Tubrication Limited | | 1977 |
| Maypole Fit Limited | Professional & scientific | 2016 |
| Maypole Health Ltd | Health & social care | 2015 |
| MB & SF Harris Limited | Other services | 2018 |
| Mbsi Classics Limited | Wholesale & retail | 2019 |
| Mcarthur Engineering Ltd | Professional & scientific | 2025 |
| MD Partners Limited | Wholesale & retail | 2000 |
| MDL Project Consulting Limited | Information & communication | 2024 |
| Mead Demolition Ltd | Construction | 2020 |
| Meadow Brook Services Ltd | Administrative & support | 2019 |
| Meadow Field Consultants Ltd | Professional & scientific | 2004 |
| Meatline (Bicknacre) Limited | Wholesale & retail | 2016 |
| Meatline (Maldon) Limited | Wholesale & retail | 2017 |
| Meatline Ltd | Wholesale & retail | 1998 |
| Media and Marketing Limited | Professional & scientific | 2004 |
| Mediavita Limited | Professional & scientific | 2014 |
| Medical Services Danbury Limited | Construction | 2008 |
| Melo Translations Limited | Professional & scientific | 2025 |
| Melrose Housekeeping Ltd | Administrative & support | 2022 |
| Melvue Limited | Administrative & support | 2019 |
| MEN in Progress Ltd | Other services | 2025 |
| Mens Barbers Ltd | Other services | 2010 |
| Mepco Engineering Limited | Professional & scientific | 2013 |
| Meraki Distribution Ltd | Wholesale & retail | 2022 |
| Meraki Moon Emporium Ltd | Wholesale & retail | 2022 |
| Meridian Windows Doors and Conservatories Limited | Construction | 2007 |
| Meridien Property Limited | Real estate | 2015 |
| Merolina Ltd | Other services | 2025 |
| Messenex Property Investments Ltd | Real estate | 2017 |
| MG & JG Holdings Limited | Administrative & support | 2004 |
| MGJ Fabrications Limited | Administrative & support | 2013 |
| Mglb Contracts Ltd | Administrative & support | 2019 |
| MHB Recruitment Limited | Administrative & support | 2025 |
| MHS Boxing Ltd | Arts & recreation | 2017 |
| Michael D'Arcy Limited | Real estate | 2001 |
| Michael Mckell Limited | Arts & recreation | 2003 |
| Michaelobrien PAINTING84 Ltd | Construction | 2024 |
| Microdata Solutions Limited | Information & communication | 2019 |
| Mid-Essex Property and Investment Company Limited | Real estate | 1975 |
| Midas Consultancy & Engineering Ltd | Professional & scientific | 2024 |
| Middleground Limited | Education | 2004 |
| Midnight Studios Ltd | Information & communication | 2025 |
| Mightdo Limited | Professional & scientific | 2016 |
| Mike Riglin Nursing Limited | Health & social care | 2015 |
| Mila Consultants Ltd | Administrative & support | 2019 |
| Milenna Homes Ltd | Construction | 2022 |
| Milestone Coaches Ltd | Transport & logistics | 2017 |
| Mill Barn Alpacas Limited | Agriculture & fishing | 2022 |
| Mill Beach Seaside Resort Limited | Hospitality | 1947 |
| Mill Road 1965 Limited | Extraterritorial | 2013 |
| Mill Road Management 43 Company Ltd | Real estate | 2024 |
| Millennium Systems Ltd. | Information & communication | 1996 |
| Millon International Freight Limited | Transport & logistics | 1976 |
| Minder Holdings Limited | Financial services | 2019 |
| Mindset Properties Ltd | Real estate | 1997 |
| Mindswan Limited | Professional & scientific | 2017 |
| Minerva OTO Ltd | Information & communication | 2023 |
| Mini Swim Stars Ltd | Education | 2022 |
| Minot Brothers Ltd | Administrative & support | 2024 |
| Missing Middle Ltd | Professional & scientific | 2022 |
| Mitchells Jewellers Limited | Wholesale & retail | 2011 |
| Mitreform Limited | Professional & scientific | 1994 |
| Mitromax Ltd | Construction | 2023 |
| Mity Ltd | Wholesale & retail | 2020 |
| MJ Brewing Limited | Hospitality | 2024 |
| MJA Engineering Limited | Professional & scientific | 2007 |
| MJO Education Limited | Education | 2018 |