| M.P.B. Marine Services Limited | Helston | 2011 |
| M.P.S. Risk Management Ltd | Inverurie | 2016 |
| M.R.H.Minerals Limited | Carlisle | 1941 |
| M.Rockliffe-Engineering Ltd | London | 2018 |
| M.S.S.J. Chatwood Holdings Ltd | Bolton | 2023 |
| M3 Mining UK Limited | London | 2025 |
| M4 Energy Services Ltd | Fraserburgh | 2024 |
| M54 Land & Minerals Management Ltd | Telford | 2018 |
| MA Fishing & Re-Entry Consultants Ltd | Montrose | 2011 |
| Mabkabo Investment Company Limited | London | 2018 |
| MAC Brownhills Quarry Limited | Walsall | 2017 |
| MAC Industries Holdings Ltd | London | 2025 |
| MAC Nuneaton Quarry Limited | Walsall | 2017 |
| MAC Rigging Services Ltd | Isle Of Lewis | 2018 |
| Mac's SY Limited | Isle Of Lewis | 2018 |
| MAC1810 Ltd. | Aberdeen | 2025 |
| Macalister Associated Consulting Ltd. | Ellon | 2010 |
| Macans Global Services Limited | Salford | 2025 |
| Macaulay Askernish Ltd. | South Uist | 1990 |
| Macaulay Plant Services Ltd | 31 York Street | 2017 |
| Maccs Energy Services & Solutions Ltd | Inverness | 2018 |
| Macd Services Ltd | Aberdeen | 2023 |
| Macfearchair Electrical Limited | Londonderry | 2018 |
| Macgregor Engineering Service Ltd | Bournemouth | 2018 |
| Macinnes Mechanical Services Ltd | Isle Of Benbecula | 2018 |
| Mackay Electrical Services Ltd | Alness | 2011 |
| Mackenzie Hughes Services Ltd | Dundee | 2019 |
| Mackenzie International Services Ltd | Montrose | 2017 |
| Mackenzie Petroleum Ltd | Montrose | 2013 |
| Mackie Inspection Services Ltd. | Edinburgh | 2010 |
| Mackinlay Well Engineering Limited | Westhill | 2013 |
| Macky's Professional NDT and Rope Access Ltd Services Limited | Oldham | 2025 |
| Maclean Completions Ltd | Aberdeen | 2025 |
| Maclellan Contractors Limited | Isle Of North Uist | 2015 |
| Macleod Construction Limited | Argyll | 1996 |
| Macoliver Group Limited | Wixams | 2017 |
| Macs Access Limited | Inverurie | 2019 |
| Macs Digger Services Limited | Leigh | 2025 |
| MADOG30 Limited | Swansea | 2020 |
| Magellanic Offshore Limited | Sutton | 2025 |
| Magheramore Quarry Ltd | Toomebridge | 2022 |
| Magic Dolphin Ltd | Helston | 2023 |
| Magma Geological Limited | Edinburgh | 2018 |
| Magma Quarry Services Ltd | Motherwell | 2017 |
| Magna Earth Minerals Limited | London | 2023 |
| Magnus Resources Limited | Crediton | 2023 |
| Magtek Design & Development Ltd | Inverness | 2013 |
| Maharajah OIL and GAS Limited | London | 2011 |
| Mahenge Resources Limited | Richmond Upon Thames | 2021 |
| Mahkuda Global Mining Ltd | London | 2025 |
| Maian Holding Ltd | Ruislip, | 2023 |
| Mainprize Asia Ltd | Scarborough | 2019 |
| Mainsprint Limited | Liverpool | 1983 |
| Mainstay North Sea Limited | Kirkwall | 2002 |
| Mainstream Engineering Ltd | Luton | 2021 |
| Maintech Engineering Services Limited | Aberdeen | 2007 |
| Maison Royale DE Appleton DE LA Marche Ltd | London | 2022 |
| Majlis Energy Limited | London | 2018 |
| Major Corporation Limited | Hertfordshire | 2023 |
| Major Drilling (NI) Limited | Omagh | 2010 |
| Major Hazards Consultancy Ltd. | Kemnay | 2016 |
| Major OIL and GAS Consultants Limited | Marlborough | 2006 |
| Makkah & AL Madinah Holdings Ltd | Preston | 2016 |
| Malco Oilfield Services Ltd | Matlock | 2017 |
| Malcolm Blaes Company Limited | Linwood Industrial Estate | 1961 |
| Maldon Salt Limited | Essex | 1926 |
| Mallusk Quarries Limited | | 1959 |
| Maltby Colliery Limited | County Durham | 2007 |
| Mama Africa Agricultural and Livestock Group COM. Limited | London | 2025 |
| Maman Burkina Ltd | Cardiff | 2025 |
| MAN on a Ledge Ltd | Aberdeen | 2014 |
| Mana Energy Limited | Paddington Central | 2015 |
| Manby BGE Limited | London | 2022 |
| Mandale Stone Company Limited | Matlock | 2001 |
| Mandall's Slate Company Limited | Kirkby In Furness | 1902 |
| Manor of Roscraddoc Limited | Bristol | 1952 |
| Manorscene Limited | York | 2014 |
| Mansa Services UK Limited | London | 2025 |
| Mansfield Sand Company Limited | Mansfield | 1999 |
| Mansgate Restoration Ltd | Rotherham | 2026 |
| Manstone Resources PLC | Oldbury | 2017 |
| Mantos Copper (UK) NO. 1 Limited | London | 2015 |
| Mantos Copper (UK) NO. 2 Limited | London | 2015 |
| Manuem Limited | Cardiff | 2023 |
| Manuli Hydraulics UK Limited | Halesowen | 2002 |
| MAP Electrical Services (UK) Limited | Blackpool | 2013 |
| MAP Inspection Services Ltd | Lower Kingswood | 2007 |
| Mapleleaf Drilling Limited | Glasgow | 2025 |
| Maracon Ltd | Bournemouth | 2014 |
| Marble Italia Limited | London | 2014 |
| Marchwood Aggregates Limited | Wolverhampton | 1993 |
| Marchwood Recycling Limited | Wolverhampton | 2012 |
| Marcore Limited | Newcastle Upon Tyne | 2022 |
| Marial Energy Consulting Ltd | Cardiff | 2023 |
| Marine Direct Consultants Limited | Westhill | 2001 |
| Marine Electrical Engineering Ltd | Montrose | 2014 |
| Marine Logistics Gold Mining Limited | Watford | 2020 |
| Marine Minerals Limited | Falmouth | 2009 |
| Marine Offshore Designers (1987) Limited | Great Yarmouth | 1987 |
| Mark Bessell Engineering & Consultancy Limited | Benfleet | 2016 |