| Mays Property Group Ltd | Construction | 2026 |
| Maze Coaching Ltd | Education | 2012 |
| Mazzinelli Ltd | Real estate | 2025 |
| MB CAD Services Ltd | Information & communication | 2015 |
| MB Cars Swindon Limited | Wholesale & retail | 2021 |
| MB Refurbishments & Building Services Ltd | Construction | 2022 |
| MB Scaff Limited | Construction | 2025 |
| MB Treads Swindon Limited | Wholesale & retail | 2021 |
| MB8SW Recovery&Transport Ltd | Transport & logistics | 2024 |
| Mbali Kiddies Corner Ltd | Wholesale & retail | 2020 |
| MBC Packaging Ltd | Professional & scientific | 2023 |
| MBG Catering Limited | Hospitality | 2022 |
| MBH Carpentry and Joinery Ltd | Construction | 2017 |
| MBH Plumbing & Heating Limited | Construction | 2014 |
| MBLSKILLUP25 Ltd | Administrative & support | 2025 |
| MBM Global Ltd | Information & communication | 2023 |
| Mbros Cash & Carry Limited | Wholesale & retail | 2012 |
| MC Automotive Consultancy Limited | Wholesale & retail | 2021 |
| MC Barbers Ltd | Other services | 2025 |
| MC City UK Ltd | Transport & logistics | 2017 |
| MC Event Loos Ltd | Administrative & support | 2026 |
| MCA 18 Ltd | Transport & logistics | 2019 |
| Mcarthur Court (Swindon) Management Company Limited | Other services | 2003 |
| MCB Distributions Ltd | Wholesale & retail | 2026 |
| MCB Logistics Limited | Transport & logistics | 2018 |
| Mcbride Catering Company Ltd | Hospitality | 2024 |
| Mcbride Project Management Services Limited | Professional & scientific | 2016 |
| Mccahill & Partners LLP | | 2024 |
| Mccartney Maxwell Limited | Administrative & support | 2003 |
| Mcck Food Ltd | Hospitality | 2020 |
| Mcctaffs Cakes Ltd | Other services | 2016 |
| Mcdonald HAY Ltd | Health & social care | 2014 |
| Mcdonald's Badminton Academy Ltd | Education | 2020 |
| Mcdowell Surveyors Limited | Professional & scientific | 2020 |
| Mcenroe Carpentry & Joinery Ltd | Construction | 2021 |
| MCF 13 Limited | Transport & logistics | 2019 |
| Mcfarlane Sales & Lettings Ltd | Real estate | 2007 |
| Mcglynn Contracts Ltd | Construction | 2016 |
| MCH Window Cleaning Ltd | Administrative & support | 2024 |
| MCK IT Ltd | Information & communication | 2008 |
| Mckay 3.3 Limited | Financial services | 2024 |
| Mckay Property Limited | Real estate | 2011 |
| Mcleod Computing Limited | Information & communication | 1998 |
| MCR RIO Ltd | Hospitality | 2025 |
| Mcraw Limited | Professional & scientific | 2020 |
| Mcshane Properties Ltd | Real estate | 2023 |
| Mcvey Clarke Financial Services | | 1992 |
| Mcxmc Ltd | Health & social care | 2024 |
| Mdbd Ltd | Professional & scientific | 2020 |
| Mdboz Ltd | Construction | 2022 |
| MDC Couriers Ltd | Transport & logistics | 2019 |
| Mdfilms Ltd | Information & communication | 2024 |
| Mdgl Ltd | Construction | 2017 |
| MDK Buildings Ltd | Construction | 2022 |
| MDK Consult Ltd | Health & social care | 2020 |
| MDR the Handy Guys Ltd | Construction | 2023 |
| MDS BOZ Limited | Construction | 2025 |
| MDU Construction Limited | Construction | 2019 |
| Mead Family Funerals Ltd | Other services | 2015 |
| Mead Marketing & Communications Ltd | Professional & scientific | 2024 |
| Meadow Hill Construction (Swindon) Ltd | Construction | 2018 |
| Meadow Plus Limited | Real estate | 2017 |
| Meadow Store Limited | Real estate | 2002 |
| Meads Business Services Ltd | Real estate | 2026 |
| Meanduni Limited | Information & communication | 2025 |
| Mear Consulting Ltd | Professional & scientific | 2019 |
| Measured Design Solutions Limited | Professional & scientific | 2019 |
| Meca Estates Ltd | Real estate | 2012 |
| Meca-Xl Limited | Arts & recreation | 2019 |
| Mecasoft Limited | Professional & scientific | 2017 |
| Mechanics' Institution Trust Enterprises Limited | Hospitality | 2017 |
| Medaesthet Ltd | Health & social care | 2026 |
| Medaesthetic Pro Ltd | Wholesale & retail | 2025 |
| Medi Tech (Southwest) Ltd | Health & social care | 2013 |
| Media 100 Limited | Professional & scientific | 2021 |
| Media IT Soft Limited | Information & communication | 2024 |
| Media Mars Ltd | Wholesale & retail | 2014 |
| Media Mirror Limited | Real estate | 2024 |
| Media Services Direct Ltd | Professional & scientific | 2013 |
| Mediadirect Services Ltd | Professional & scientific | 2024 |
| Medialadd Ltd | Professional & scientific | 2023 |
| Medical Media (Haxton) Limited | Professional & scientific | 2006 |
| Medical Practice Management (Swindon) Limited | Administrative & support | 2018 |
| Medicine Events C.I.C. | Arts & recreation | 2019 |
| Medicobay Ltd | Health & social care | 2018 |
| Medion Electronics Limited | Wholesale & retail | 1998 |
| Mediwellness Ltd | Health & social care | 2024 |
| Medmech Limited | Construction | 2003 |
| Mednest Limited | Health & social care | 2023 |
| Medrano CAD Limited | Professional & scientific | 2007 |
| Medsec Training & Security Solutions Ltd | Administrative & support | 2025 |
| Meekman Ltd | Health & social care | 2022 |
| Meera OM Limited | Wholesale & retail | 2026 |
| Meet and EAT (Southampton) Ltd | Hospitality | 2024 |
| Meet My Ancestors Limited | Manufacturing | 2024 |
| Mega Discounts Swindon Ltd | Administrative & support | 2025 |
| Mega Grand Food Limited | Other services | 2019 |
| Megahightec Limited | Information & communication | 2016 |
| Megron Tech Ltd | Information & communication | 2010 |
| Mehdi's Full Valeting Service Ltd | Administrative & support | 2023 |