| Mereland Land Company Limited | Extraterritorial | 1995 |
| Merri Estates Limited | Real estate | 2025 |
| Merritt Cleaning Ltd | Administrative & support | 2023 |
| Metallic Impressions Limited | Manufacturing | 2025 |
| Metamorfix Limited | Professional & scientific | 2002 |
| Mews Mead Management Co. Limited | Real estate | 1992 |
| Mezze Food Ltd | Hospitality | 2026 |
| MFC529 Ltd | Households | 2017 |
| MG Rover Group Limited | Manufacturing | 2023 |
| MGB Projects Ltd | Professional & scientific | 2019 |
| MGG Southern Ltd | Wholesale & retail | 2026 |
| MGL Consultancy Ltd. | Information & communication | 2018 |
| MIB Cleaning Ltd | Administrative & support | 2023 |
| Michael Fiddy Roofing Ltd | Construction | 2024 |
| Michael Ring Consulting Limited | Information & communication | 2017 |
| Micky Productions Ltd | Construction | 2018 |
| Middleton Cable Jointing Limited | Construction | 2018 |
| Middleton Insurance Services Ltd | Financial services | 2004 |
| Middleton Place Residents Association Limited | Households | 1982 |
| Middleton Sports Club Limited(The) | Arts & recreation | 1946 |
| Middleton-On-Sea (Beach Huts) Limited | Other services | 1978 |
| Midhurst Garage Limited | Wholesale & retail | 2010 |
| Migdalcare Ltd | Health & social care | 2024 |
| Mike Car Repair Limited | Manufacturing | 2019 |
| Mike Collier Composites Limited | Manufacturing | 2023 |
| Mike Denton Stunts Limited | Arts & recreation | 2022 |
| Milk and Metal Ltd | Professional & scientific | 2020 |
| Millennium House (Freehold) Limited | Extraterritorial | 2014 |
| Miller Construction Sussex Limited | Construction | 2020 |
| Milo and MAE Ltd | Wholesale & retail | 2026 |
| Mindfirst Education Ltd | Health & social care | 2024 |
| Mindwear Ltd | Wholesale & retail | 2025 |
| Minerva Place Residents Limited | Real estate | 2010 |
| Mini Market Bognor Ltd | Wholesale & retail | 2018 |
| Minivault Ltd | Wholesale & retail | 2025 |
| Minor Limited | Real estate | 1990 |
| Minters Paving Limited | Construction | 2020 |
| Minx Tailoring Limited | Wholesale & retail | 2005 |
| Mishean Limited | Wholesale & retail | 2025 |
| Mitchell's Travel Ltd | Administrative & support | 2019 |
| Mixmate Ltd | Construction | 1995 |
| MJR Solicitors Limited | Professional & scientific | 2017 |
| ML Bespoke Bathrooms Ltd | Construction | 2025 |
| MLF Productions Ltd | Information & communication | 2019 |
| MLW Land Limited | Real estate | 2014 |
| MM Wach Ltd | Households | 2025 |
| Mm@P Ltd | Information & communication | 2024 |
| MNY Electrical Ltd | Extraterritorial | 2023 |
| Moan Real Estate Ltd | Real estate | 2025 |
| Mobile Stage Limited | Arts & recreation | 2011 |
| Mobility and Comfort Limited | Wholesale & retail | 2011 |
| Mobility at Sea Limited | Health & social care | 2013 |
| Modern & Classic Interiors Limited | Administrative & support | 2010 |
| Modern Lifestyle Ltd | Wholesale & retail | 2023 |
| MOJ Nile Limited | Manufacturing | 2016 |
| Mollivita Limited | Wholesale & retail | 2022 |
| Monkey Nuts (About Cleaning) Limited | Administrative & support | 2020 |
| Monstamonsta Ltd | Wholesale & retail | 2026 |
| Moody Hound Ltd | Construction | 2025 |
| Morgan Property Services (Sussex) Limited | Other services | 2022 |
| Morshead Consulting Limited | Professional & scientific | 2019 |
| Mortgage and Protection Advice Centre Ltd | Financial services | 2021 |
| MOTIV8 Connect Group Ltd | Administrative & support | 2023 |
| MOTIV8-SOLUTIONS for Life Limited | Administrative & support | 2006 |
| Motor NET Service & Repairs Limited | Extraterritorial | 2006 |
| Mount Pleasant Kingsway RTM Company Limited | Households | 2020 |
| Mountbureau Limited | Hospitality | 2015 |
| Mountbureau Property Ltd | Real estate | 2021 |
| Moxey Creative Ltd | Professional & scientific | 2025 |
| MP Tech Solutions Limited | Professional & scientific | 2021 |
| Mpdf Buildings Ltd | Construction | 2019 |
| Mphlace Ltd | Manufacturing | 2015 |
| MPL Demerger Limited | Real estate | 2018 |
| MPL NEWCO1 Limited | Financial services | 2025 |
| MPL NEWCO2 Limited | Financial services | 2025 |
| MS Maintenance & Repairs Ltd | Construction | 2025 |
| MSC Plus Limited | Professional & scientific | 2005 |
| MSF Southern Ltd | Hospitality | 2022 |
| MSP Details Ltd | Construction | 2018 |
| MT Plumbing & Heating (Sussex) Ltd | Construction | 2025 |
| Multi-Use Materials Supplies Ltd | Wholesale & retail | 2026 |
| Multifruit Marketing Limited | Wholesale & retail | 2025 |
| Murrell Arms Limited | Hospitality | 2019 |
| Murrell Gardens Management Company Ltd | Real estate | 2012 |
| Music for Health Limited | Health & social care | 2003 |
| Mustards Restaurants Limited | Hospitality | 2018 |
| MW Motorcycles Ltd | Wholesale & retail | 2024 |
| Mwlb Limited | Construction | 2020 |
| My Garden Plan Ltd | Information & communication | 2025 |
| My Ice Identity Limited | Public administration | 2026 |
| My Life Films Limited | Health & social care | 2014 |
| My Tech Planner Limited | Wholesale & retail | 2023 |
| N C H Landscapes Limited | Administrative & support | 2015 |
| N Gauge Trains Ltd | Wholesale & retail | 2025 |
| N K Installations Ltd | Construction | 2012 |
| N P Autos Limited | Wholesale & retail | 2010 |
| N2PEOPLESKILLS Ltd | Arts & recreation | 2014 |
| Nagerrek LLP | | 2021 |
| Nails by Jade Ltd | Other services | 2024 |
| Nara Healthcare Limited | Health & social care | 2013 |