| Mortimer, Harley Limited | Real estate | 1920 |
| Moss Close Limited | Extraterritorial | 2013 |
| Motor Experts by MGN UK Limited | Wholesale & retail | 2026 |
| Motormastery Ltd | Wholesale & retail | 2025 |
| Movement in Media Limited | Information & communication | 2006 |
| Moverve Limited | Information & communication | 2006 |
| MPA Contract Services Ltd | Administrative & support | 2012 |
| Mpdg Ltd | Manufacturing | 2025 |
| MR Carriages Limited | Transport & logistics | 2023 |
| MR Hands Ltd | Construction | 2024 |
| MRC Works Limited | Other services | 2022 |
| MRS Willoughby Productions Limited | Information & communication | 2015 |
| MSC Refurbishments Ltd | Construction | 2021 |
| Mscc Holdings Limited | Real estate | 2025 |
| Msjv Consultancy Ltd | Professional & scientific | 2025 |
| Msky Property Ltd | Real estate | 2011 |
| MT & JT Holdings Limited | Real estate | 2021 |
| MT & JT Investments Ltd | Real estate | 2017 |
| MT4 Services Ltd | Real estate | 2025 |
| MTB Properties (Hertfordshire) Ltd | Real estate | 2021 |
| Muckish Limited | Professional & scientific | 2022 |
| Mudra Limited | Real estate | 2022 |
| Mullyfa Investments Limited | Real estate | 2021 |
| Multi-Grind Services Limited | Manufacturing | 1977 |
| Mumina Enterprises Ltd | Real estate | 2019 |
| Mumtessori Limited | Education | 2024 |
| Munky Design Ltd | Information & communication | 2002 |
| Murphy & Co Builders Ltd | Construction | 2024 |
| Murphy Family Ltd | Financial services | 2025 |
| Murray Road (Northwood) Ltd | Real estate | 2023 |
| Murrell Estates Limited | Construction | 1967 |
| Murrell's Construction and Landscaping Limited | Administrative & support | 2020 |
| Musgrove Plumbing Limited | Construction | 2025 |
| Muswell Hill Development Ltd | Construction | 2023 |
| MW Services (Herts) Limited | Construction | 2021 |
| MWJ Ltd | Other services | 2012 |
| My Baby Sleep Plan Limited | Other services | 2025 |
| My Coffee Affair Ltd | Hospitality | 2023 |
| My Property Investments Limited | Real estate | 2004 |
| Myrb Properties Limited | Real estate | 2016 |
| Myrtle Management Company Limited | Administrative & support | 2006 |
| Mysa Consultancy Limited | Hospitality | 2016 |
| Mysa Global Ltd | Hospitality | 2018 |
| Myself Contained Ltd | Transport & logistics | 2023 |
| Mytomes Limited | Wholesale & retail | 2026 |
| N B C S Limited | Real estate | 1991 |
| N&L Financial Solutions Limited | Professional & scientific | 2011 |
| N. Bywater Roofing Limited | Construction | 2025 |
| N.I.S. Consulting Limited | Professional & scientific | 2020 |
| N.I.S. Investments Limited | Financial services | 2021 |
| Naatap Holdings Limited | Financial services | 2019 |
| Nagle Mechanical Services Ltd | Construction | 2015 |
| Nails Art & Beauty Ltd | Other services | 2025 |
| Namno Music Ltd | Arts & recreation | 2021 |
| Nanny Littons Coffee POT Ltd | Hospitality | 2024 |
| Nanny Littons Limited | Hospitality | 2019 |
| Nanny Littons Lunch Run Ltd | Hospitality | 2019 |
| Nano Personal Care Limited | Wholesale & retail | 2019 |
| Narayan IT Ltd | Information & communication | 2015 |
| Nardelli Gaming Limited | Information & communication | 2024 |
| Nardelli Investment Ltd | Real estate | 2018 |
| Nascot Developers Ltd | Real estate | 2022 |
| Nathan Slinger Scenery Ltd | Information & communication | 2019 |
| Nathar Consultancy Services Limited | Information & communication | 2022 |
| National Association of Public and Proprietary Golf Clubs and Courses Limited | Arts & recreation | 2015 |
| National Honey Show Limited(The) | Education | 1932 |
| Natural Elements UK Limited | Administrative & support | 2007 |
| Natural Medical Clinic Ltd | Health & social care | 2018 |
| Nature Heals 2025 Ltd | Hospitality | 2024 |
| Navent Limited | Hospitality | 2013 |
| Navigator Corporate Services Limited | Professional & scientific | 2003 |
| Nayur Investment Solutions Limited | Real estate | 2012 |
| Neat Elite Ltd | Arts & recreation | 2022 |
| Neilsun Management Ltd | Wholesale & retail | 2017 |
| Nejad Ltd | Information & communication | 2019 |
| Nelday Flat Management Limited | Real estate | 1996 |
| Neodiagnostic Research Limited | Professional & scientific | 1999 |
| Neru Night Ltd | Manufacturing | 2026 |
| Nest Construction Services Ltd | Construction | 2025 |
| Netmedia International UK Limited | Information & communication | 2009 |
| Netmums Limited | Information & communication | 2002 |
| Nettalk S a L Ltd | Wholesale & retail | 2022 |
| Netwire Contracts Ltd | Construction | 2012 |
| Netzero Initiatives Ltd | Professional & scientific | 2025 |
| Neugenis Limited | Other services | 2015 |
| Neuro Spicy Ltd | Professional & scientific | 2023 |
| Neuroverse Ltd | Health & social care | 2024 |
| Neurowaze Ltd | Information & communication | 2025 |
| New City Dating Ltd | Information & communication | 2025 |
| New Country Properties Limited | Real estate | 2017 |
| New Court (Croxley) Management Limited | Households | 1989 |
| New Dawn Coaching and Careers Ltd | Professional & scientific | 2025 |
| New Dawn Solar Ltd | Other services | 2020 |
| New Gang Ltd | Real estate | 2015 |
| New Method Company Limited | Health & social care | 2026 |
| New Road Croxley Developments Ltd | Real estate | 2023 |
| New Road Dental Limited | Health & social care | 2007 |
| New Road Service Station Croxley Limited | Wholesale & retail | 2022 |
| New Ventures Property Ltd | Real estate | 2015 |
| New Zealand Natural Foods Limited | Professional & scientific | 2005 |