| MND UK North Sea Ltd | London | 2024 |
| MNS Mining Limited | West Glamorgan | 2009 |
| Mobay Chester Holding Company Limited | London | 2026 |
| Mobility OIL and GAS Limited | Borehamwood | 2010 |
| Modec UK Ltd. | London | 2017 |
| Moderntronics Limited | Harrow | 1999 |
| Moduspec Ltd | Westhill | 1988 |
| Moeserv Technical Services Ltd | Aberdeen | 2013 |
| Moffitt QSE Ltd | Newcastle Upon Tyne | 2013 |
| MOH & MOG Limited | Reading | 2025 |
| Mojolan Services Limited | Edinburgh | 2016 |
| Mold Investments Ltd | Coventry | 2015 |
| Monaco Engineering Solutions Limited | Leatherhead | 2006 |
| Monash Mining Limited | London | 2023 |
| Mondial Marmi Ltd | Pinner | 2021 |
| Mone Brothers Excavations Limited | Leeds | 1972 |
| Monel Energy 2 Limited | Burton-On-Trent | 2024 |
| Monel Energy Limited | Burton-On-Trent | 2024 |
| Monitor Exploration Limited | Teddington | 2018 |
| Monkey Park Community Ltd | St. Ives | 2023 |
| Montage Management Services Ltd | Coventry | 2020 |
| Montana Services Limited | Aberdeen | 2007 |
| Monterrico Metals Limited | London | 2001 |
| Montrose Industries Limited | Leominster | 1960 |
| Monument Engineering Ltd | Washington | 2020 |
| Moore & Sons Limited | Ibstock | 1911 |
| Moorhay Stone and Slate Quarry Ltd | Chesterfield | 1997 |
| Moorhouse Petroleum Limited | Bridlington | 1994 |
| Mooring Systems Limited | Aberdeen | 2003 |
| Moray Firth Offshore Ltd | Elgin | 2017 |
| Moray Global Limited | Aberdeen | 2015 |
| Moreton C.Cullimore(Gravels)Limited | Gloucester | 1943 |
| Morgan Corporate Limited | London | 2015 |
| Morgan James Bespoke Ltd | Kirkcaldy Fife | 2019 |
| Morgandica Ltd | Beckenham | 2023 |
| Morgenstern Energy Ltd | Edinburgh | 2018 |
| Moro Nleyo Ltd | Johnstone | 2025 |
| Morridon Electrical Limited | Inverness | 2007 |
| Morvent Mining Limited | Plymouth | 2009 |
| Motapa Mining Company UK Limited | London | 2021 |
| Mott MAC Supplier Ltd | London | 2019 |
| Mount Avenue Limited | London | 2021 |
| Mount Kearney Quarries & Contracts Ltd | Newry | 2020 |
| Mountskape Limited | Altrincham | 2015 |
| Mourne Aggregates (N.I.) Limited | Derryogue Road | 1988 |
| Moxico Kosovo Limited | London | 2018 |
| Moxico Resources PLC | London | 2013 |
| Moyard Properties Limited | Belfast | 2006 |
| MPF Services UK Ltd | Roslin | 2020 |
| Mpfm Limited | Choppington | 2008 |
| MPH Engineering (OIL & GAS) Ltd | Stockton On Tees | 2012 |
| MPH Plant Hire Ltd | Rotherham | 2026 |
| MPJ E&I Limited | Gourock | 2020 |
| MQ Subsea Engineering Services Ltd | Montrose | 2016 |
| MR Inspection Services Limited | Aberdeen | 2017 |
| MRC International Ltd | London | 2018 |
| MS Engineering Solutions Ltd | Montrose | 2020 |
| Ms&A Consulting Limited | Kirkwall | 2011 |
| Msampa Mining Ltd | Oxford | 2016 |
| Msbv Limited | Aberdeen | 2013 |
| MSC Global Group Ltd | London | 2024 |
| Mscm Limited | High Wycombe | 1999 |
| Mscm Services Ltd | Dunblane | 2022 |
| MSD Electrical Engineering Ltd | Stockton-On-Tees | 2015 |
| MSG Inspections Limited | Blackpool | 2020 |
| MSL Tech Ltd | Aberdeen | 2019 |
| Mspc Engineering Ltd | Stockton-On-Tees | 2025 |
| MT Petrochemical Services Ltd | Louth | 2022 |
| MTM Competency Ltd | Chesterfield | 2014 |
| MTS Engineering Solutions Ltd | Ilford | 2021 |
| MTS Groundworks Ltd | Wrexham | 2018 |
| MTS Offshore Surveying Limited | Sunderland | 2012 |
| MTW Project Services Ltd | Motherwell | 2015 |
| Muir Rope Access Ltd | Forfar | 2025 |
| Muirhouse Ventures Limited | London | 2022 |
| Multi - AGG Limited | Marlborough | 1984 |
| Multi Connections Ltd | Aberdeen | 2019 |
| Mumum Carly Ltd | London | 2023 |
| Munro Engineering Designs Limited | Inverurie | 2006 |
| Munro Projects Ltd | Sutherland | 2022 |
| Muqiao Ltd | West Midlands, England | 2024 |
| Murchie Aggregates Ltd | Isle Of Arran | 2023 |
| Murdo's Mechanics Ltd | Peterhead | 2015 |
| Murphy Petroleum Limited | London | 1964 |
| Murphy Schiehallion Limited | London | 2012 |
| Murphy Subsea Consulting Limited | Potterton | 2016 |
| Murray & Roberts South America Limited | Swindon | 2009 |
| Murray Mountain Ltd | London | 2016 |
| Murray OIL & GAS Ltd | Morpeth | 2019 |
| Murray's Project Services Ltd | Darlington | 2011 |
| Murzuq OIL Services Limited | London | 2020 |
| Mutarazi Resource Company Limited | Leicester | 2024 |
| Mutora Ltd | Killin | 2021 |
| MV Engineering Ltd | Troon | 2025 |
| MV Martin Limited | Petworth | 2020 |
| MW Future Limited | Horsham | 2017 |
| MW Marshall Qa/Qc Ltd | Douglas | 2017 |
| Mwenke and Associates Ltd | London | 2020 |
| MXA Scandinavia Limited | Northampton | 2010 |
| Myco Energy Limited | Aberdeen | 2025 |