| MBDA UK Limited | Stevenage | A rating |
| MBI Inc. | Chessington | A rating |
| MBM Consulting Ltd | London | A rating |
| MC General Services (UK) Limited | London | A rating |
| McAfee Security UK Limited | Globe Park | A rating |
| MCAP Global Finance (UK) LLP | London | A rating |
| MCap Global Finance Limited | London | A rating |
| McCain Foods (GB) Ltd | Scarborough | A rating |
| McCann Erickson Advertising Ltd | London | A rating |
| McCann Erickson EMEA Ltd | London | A rating |
| McCann-Erickson Healthcare UK Limited | London | A rating |
| McCarthy Tetrault | London | A rating |
| McChrystal Group Limited | London | A rating |
| McCloskey International Ltd | Coalisland | A rating |
| MCCLOUD PROFESSIONAL SERVICES LTD | Shropshire | A rating |
| McCormick (UK) Ltd | Haddenham | A rating |
| MCD Global Franchising Limited | London | A rating |
| MCDERMOTT ENERGY SOLUTIONS (UK) LIMITED | London | A rating |
| McDermott Marine Construction Limited | London | A rating |
| McDermott Will & Schulte UK LLP | London | A rating |
| McDonald's Restaurants Limited | London | A rating |
| MCF Ltd | London | A rating |
| McGraw Hill Education (UK) Limited | London | A rating |
| McGuirewoods London LLP | London | A rating |
| McKesson Strategic Services Ltd | London | A rating |
| McKinnon Harris UK Ltd | London | A rating |
| McKinsey & Company Inc. United Kingdom | London | A rating |
| MDA SATCONN UK LTD | Hampshire | A rating |
| MDA Space and Robotics Limited | Didcot | A rating |
| MDNA Life Sciences (UK) Ltd | Newcastle Upon Tyne | A rating |
| MDPI UK LIMITED | Manchester | A rating |
| MDS Aero Support Limited | Derby | A rating |
| MDSOL Europe Ltd | Hammersmith | A rating |
| Meaden & Moore International LLP | London | A rating |
| MEASURABL LIMITED | Willingham | A rating |
| Mecalux UK Ltd | Birmingham | A rating |
| Mechadyne International Limited | Kirtlington | A rating |
| MECM Limited | London | A rating |
| Medallia Limited | London | A rating |
| Medallion Arts Limited | East Grinstead | A rating |
| Medatech UK Ltd | Chandlers Ford | A rating |
| Medecins Sans Frontieres (MSF) UK | London | A rating |
| MedExpress Enterprises Ltd | London | A rating |
| Medfac UK Ltd | Wellingborough | A rating |
| Mediabrands EMEA Ltd | London | A rating |
| Mediakeys Ltd | Wokingham | A rating |
| MediaMonks London Limited | London | A rating |
| Mediamorph Europe, Ltd | Chichester | A rating |
| Mediaocean Systems Ltd | London | A rating |
| Mediaplanet Limited | London | A rating |
| MedImmune Limited | Cambridge | A rating |
| Mediobanca S.p.A London Branch | London | A rating |
| Mediterranean Shipping Company (UK) Ltd | Ipswich | A rating |
| Medpace UK Limited | London | A rating |
| Medreich PLC | Feltham | A rating |
| Medtronic Limited | Watford | A rating |
| MEG Derby Limited | Foston | A rating |
| Mega International Commercial Bank | London | A rating |
| Megaport (UK) Limited | London | A rating |
| MEGGER INSTRUMENTS LIMITED | DOVER | A rating |
| Meiji UK Limited | Birmingham | A rating |
| Meiji Yasuda Europe Ltd | London | A rating |
| Meinhardt (UK) Limited | London | A rating |
| MeiraGTX UK II Limited | London | A rating |
| Meiyume (UK) Limited t/a Meiyume | Trowbridge | A rating |
| Meketa Investments London Ltd. | London | A rating |
| Melcombe Partners Ltd | London | A rating |
| MELLORS GROUP LTD | NOTTINGHAM | A rating |
| Meltwater (UK) Limited | London | A rating |
| Memrise Ltd | London | A rating |
| Menai Finance (UK) Ltd. | London | A rating |
| Menar Holdings Limited | London | A rating |
| Menicon Limited | Northampton | A rating |
| Meniga Limited | London | A rating |
| Menlo Security Limited | Bracknell | A rating |
| Mentor IMC Ltd | London | A rating |
| Menwell Limited | London | A rating |
| Menzies Aviation Holdings Ltd | Hounslow | A rating |
| Merative UK Ltd | Preston | A rating |
| Mercaux Limited | London | A rating |
| Mercedes AMG High Performance Powertrains Ltd | Northampton | A rating |
| Mercedes-Benz Cars UK Limited | Milton Keynes | A rating |
| Mercedes-Benz Financial Services UK Ltd | Milton Keynes | A rating |
| Mercer Engineering Ltd | Dereham | A rating |
| Mercer Ltd | London | A rating |
| Mercer Risk Management Limited | London | A rating |
| MerchantCantos | London | A rating |
| Merck Life Science UK Limited | Gillingham | A rating |
| Merck Sharp & Dohme (UK) Limited | London | A rating |
| Mercor Innovations UK Ltd | London | A rating |
| Mercur Solutions (UK) Ltd | Bracknell | A rating |
| MERCURIA UK LLP | London | A rating |
| Mercurius IT Limited | Milton Keynes | A rating |
| Mercury Computer Systems Limited | Reading | A rating |
| Mercury Engineering & Building Services Limited | London | A rating |
| Mercy Corps Europe | Edinburgh | A rating |
| Meren Services Limited | London | A rating |
| Mergermarket Limited t/a Acuris | London | A rating |
| Meridian Lightweight Technologies UK Ltd | Sutton-in-Ashfield | A rating |
| Meridian Technique Limited | Southampton | A rating |
| MERIL UK LIMITED | CRAWLEY | A rating |
| Merit Group Plc | London | A rating |
| Meritor Heavy Vehicle Braking Systems (UK) Limited | Cwmbran | A rating |
| Merkle UK Three Ltd | London | A rating |
| Merkur Casino Holdings UK Limited | Milton Keynes | A rating |
| Merlin Entertainments Group Limited | London | A rating |
| MerQube UK Limited | London | A rating |
| Merrick & Company UK Limited | Welwyn Garden City | A rating |
| Merson Signs Ltd | Glasgow | A rating |
| Merz Aesthetics UK Limited | Hemel Hempstead | A rating |
| Mesirow Financial International UK, Limited | London | A rating |
| Mesuro Limited | Pencoed | A rating |
| MET T&S LIMITED | London | A rating |
| Metal Yapi (UK) Limited | London | A rating |
| Metaldyne International (UK) Limited | Halifax | A rating |
| Metalock Engineering UK Limited | Coventry | A rating |
| Meteor Inkjet Limited | Harston | A rating |
| Meter Mix Systems Ltd | Rushden | A rating |
| Methode Electronics UK Ltd | Burnley | A rating |
| Metlen Energy & Metals S. A. UK Branch | London | A rating |
| MetLife Europe Services Limited | Brighton | A rating |
| MetraWeather (UK) Limited | Reading | A rating |
| METREA MANAGEMENT LIMITED | London | A rating |
| Metric Search Limited | Nottingham | A rating |
| MetricStream UK Limited | London | A rating |
| Metris Energy Ltd | London | A rating |
| MetrixLab UK Limited | LONDON | A rating |
| Metrol Technology Ltd | Aberdeen | A rating |
| METRON ENERGY APPLICATIONS S.A. | Immingham | A rating |
| Metropress Limited t/a Auction Technology Group | London | A rating |
| Metsa Wood UK Limited | Boston | A rating |
| Mettler Toledo Safeline X-ray Limited | Royston | A rating |
| Mettler-Toledo Ltd | Leicester | A rating |
| Mews Systems Limited | London | A rating |
| Mexichem Speciality Compounds Limited | Melton Mowbray | A rating |
| Mexichem Specialty Compounds Limited | Melton Mowbray | A rating |
| MEXICHEM UK LIMITED (t/a Koura) | Cheshire | A rating |
| MF Sweden Limited | Warwick | A rating |
| MFE Advertising UK | London | A rating |
| MFS International (UK) Ltd | London | A rating |
| MG Motor UK Ltd | Birmingham | A rating |
| MGAC LLP | London | A rating |
| MGC Pharma (UK) Ltd | London | A rating |
| MHA Advisory Limited | Milton Keynes | A rating |
| MHA Audit Services LLP | Milton Keynes | A rating |
| MHA Service Ltd | Milton Keynes | A rating |
| MHP Consulting UK Limited | Reading | A rating |
| MHWirth UK Limited | Aberdeen | A rating |
| Mi-3 Limited | Blackburn | A rating |
| MIC General Services UK limited | London | A rating |
| Michael Kors UK Limited | London | A rating |
| Michael Page International Recruitment Ltd | London | A rating |
| Micheldever Group Limited | Winchester | A rating |
| Michelin Tyre plc | Stoke-on-Trent | A rating |
| Micro AGI Limited | London | A rating |
| MICROCHIP LIMITED | Wokingham | A rating |
| Microland Limited | London | A rating |
| Micromass UK Limited | Wilmslow | A rating |
| Micron Europe Limited | Bracknell | A rating |
| MICRON RESEARCH LIMITED | ELY | A rating |
| Micron Sprayers Limited | Bromyard | A rating |
| Microsoft Limited | Reading | A rating |
| Microsoft Research Limited | Cambridge | A rating |
| MicroStrategy Ltd | Chiswick | A rating |
| Midasplayer.com Limited | LONDON | A rating |
| Middle East Airlines - Air Liban | London | A rating |
| Middle East News UK Limited | London | A rating |
| Midea Home Appliances UK Ltd | Manchester | A rating |
| Miebach Consulting Limited | Abingdon | A rating |
| Miele Company Limited | Abingdon | A rating |
| MightyHive Limited | London | A rating |
| MIKI TRAVEL LIMITED | LONDON | A rating |
| MikMak International Limited | Reading | A rating |
| Milbank LLP | London | A rating |
| MILLENNIUM & COPTHORNE HOTELS LTD | LONDON | A rating |
| Millennium Capital Management Limited | London | A rating |
| Millennium Europe Limited | London | A rating |
| Miller Insurance Services LLP | London | A rating |
| Millicom Services UK Ltd | London | A rating |
| Milliken Industrials Ltd | Wigan | A rating |
| Milliman Financial Strategies Ltd | London | A rating |
| Milliman LLP | London | A rating |
| Milltek Sport Ltd | Derby | A rating |
| Milton & King Ltd | Hove | A rating |
| Milvik UK Limited | London | A rating |
| Mimecast Services Limited | London | A rating |
| Minamoto Kitchoan Europe Ltd | London | A rating |
| Mind Candy Ltd | London | A rating |
| Mind Gym Plc | London | A rating |
| MINDBODY Limited | London | A rating |
| MindPortal Ltd | London | A rating |
| Mindray (UK) Limited | Huntingdon | A rating |
| Mindteck UK Limited | Fleet | A rating |
| Mindvalley UK Group Ltd | London | A rating |
| MineGeoTech UK Limited | London | A rating |
| Minel Cosmetic Limited | London | A rating |
| Minety Battery Storage Limited | Reading | A rating |
| Minibar (UK) Limited | Burnham | A rating |
| Miniclip (UK) Limited | London | A rating |
| MINMETALS (U.K) LIMITED | LONDON | A rating |