| Mansell Court Management Limited | Households | 1980 |
| Mansell Mctaggart Group Holdings Limited | Administrative & support | 2022 |
| Manser Hunot Accountants Limited | Professional & scientific | 2015 |
| MAP Training Limited | Other services | 2015 |
| Marine Surveys and Design Limited | Professional & scientific | 2014 |
| Marketing Stop Limited | Professional & scientific | 2009 |
| Marot Limited | Transport & logistics | 2026 |
| Martha & Lawrence Ltd | Administrative & support | 2024 |
| Martin Green Media Ltd | Professional & scientific | 2025 |
| MAS Marketing Consultants Limited | Professional & scientific | 2017 |
| Mash Events Steyning Ltd | Hospitality | 2023 |
| Master Mallets Ltd | Construction | 2026 |
| Masters Accommodation Ltd | Hospitality | 2024 |
| Masters of Motion Ltd | Information & communication | 2014 |
| Maternity Networks ORG Limited | Education | 2025 |
| Mattering Limited | Arts & recreation | 2025 |
| Matthew Roberts Carpentry Limited | Manufacturing | 2016 |
| Mavericksoft Solutions Ltd | Information & communication | 2025 |
| MAY Travel Content & Marketing Limited | Professional & scientific | 2020 |
| Mayflower Media Limited | Information & communication | 2011 |
| MCA Healthcare Limited | Administrative & support | 2020 |
| Mcclelland Music Ltd. | Wholesale & retail | 2001 |
| Mccoosh Electrical Limited | Construction | 2014 |
| MCD Brickworks Limited | Construction | 2021 |
| Mchugh Civil Engineering Limited | Construction | 2010 |
| Mchugh Concrete Limited | Manufacturing | 2017 |
| MCL Software & IT Limited | Information & communication | 2023 |
| MCL Telecommunications Group Ltd | Wholesale & retail | 2002 |
| MCL Telecoms Limited | Wholesale & retail | 2014 |
| Mclellan Finance Consultancy Limited | Professional & scientific | 2020 |
| Mcmscaffolding Ltd | Construction | 2020 |
| Mcstravick Consulting Limited | Professional & scientific | 2010 |
| Meadow View (Sayers Common) Residents Company Limited | Administrative & support | 2007 |
| Mechanical Services BG Ltd | Construction | 2018 |
| Medi Property Limited | Construction | 2016 |
| Media BC Limited | Administrative & support | 2015 |
| MEDIA3I Limited | Information & communication | 2003 |
| Medibiz Ltd | Wholesale & retail | 2025 |
| Medland Associates Limited | Construction | 2010 |
| Mefomp Ltd | Health & social care | 2019 |
| MEG & Milo Ltd | Wholesale & retail | 2020 |
| Mekem Marine Ltd | Professional & scientific | 2008 |
| MEL Larsen & Associates Ltd | Professional & scientific | 2021 |
| Melo Digital Ltd | Information & communication | 2026 |
| Merkel Consulting Limited | Professional & scientific | 2026 |
| Mert Yaman Ltd | Hospitality | 2023 |
| Metaflux (UK) Limited | Wholesale & retail | 1988 |
| Meto Trading Ltd | Professional & scientific | 2012 |
| Mews Automotive Ltd | Wholesale & retail | 2021 |
| Mexmast Limited | Administrative & support | 1991 |
| MG Hot Tubs Ltd | Other services | 2024 |
| MHJ Engineering Ltd | Construction | 2018 |
| Mibookkeeping & Accountancy Limited | Professional & scientific | 2018 |
| Micro NAV (Holdings) Limited | Professional & scientific | 1995 |
| Micro NAV (Software) Limited | Information & communication | 1988 |
| Microcement Aesthetics Limited | Construction | 2021 |
| MID Sussex Angling Ltd. | Arts & recreation | 2005 |
| MID Sussex International Limited | Transport & logistics | 2024 |
| MID Sussex Lettings Ltd | Real estate | 2021 |
| MID Sussex Mortgages Limited | Financial services | 2016 |
| MID Sussex Radio CIC | Information & communication | 2016 |
| MID Sussex Super Cream Limited | Hospitality | 2025 |
| Mille Saisons Ltd. | Manufacturing | 2014 |
| Milo Flight Training Limited | Transport & logistics | 2025 |
| Mind Body Energy Limited | Health & social care | 2022 |
| Mind Logix Limited | Information & communication | 2023 |
| MINOSU68 Ltd | Hospitality | 2020 |
| Mira Build Ltd | Households | 2026 |
| Miraculous Engineering Limited | Construction | 2009 |
| Mirami Limited | Information & communication | 2023 |
| Mirfield & Smith Limited | Wholesale & retail | 1966 |
| Mirosh Ltd | Hospitality | 2022 |
| Mirtia Ltd | Wholesale & retail | 2019 |
| Miszu IT Limited | Information & communication | 2008 |
| MK Active Limited | Health & social care | 2024 |
| MLF Financial Services Limited | Financial services | 2013 |
| MLP Electrics Limited | Construction | 2024 |
| MM Travel Consulting Limited | Professional & scientific | 2021 |
| MMA Mcguinness Motors & Accessories Ltd | Wholesale & retail | 2020 |
| Mmxnson Ltd | Other services | 2025 |
| MN Engineering Developments Limited | Real estate | 2019 |
| Mnesic Group Ltd | Hospitality | 2024 |
| Mobifi Applications Limited | Information & communication | 2009 |
| Mobifi Messaging Limited | Information & communication | 2009 |
| Mobile Cellular Ltd | Extraterritorial | 2015 |
| Mobiletyrefituk Ltd | Wholesale & retail | 2024 |
| Modular Connected Solutions Ltd | Information & communication | 1998 |
| Mogll Foods Ltd | Manufacturing | 2026 |
| Momentum Homes Limited | Real estate | 1999 |
| Money MR Limited | Financial services | 2025 |
| Monteley Limited | Real estate | 2017 |
| Moor Creative Limited | Professional & scientific | 2007 |
| Moose Pictures Ltd | Information & communication | 2025 |
| Moto Union Limited | Real estate | 2025 |
| Motorcycle Escort Team Limited | Transport & logistics | 2011 |
| Mouseface Productions Limited | Administrative & support | 2020 |
| Movement & Metabolism Ltd | Health & social care | 2024 |
| MT Managem£Nt Ltd | Professional & scientific | 2019 |
| Mtech Mobile Ltd | Wholesale & retail | 2023 |
| Mulsanne Systems Ltd | Information & communication | 2025 |