| Clucaidh Holdings Ltd | Lairg | 2023 |
| Cluff Offshore Holdings Limited | London | 2022 |
| Clyde Planning & Project Controls Ltd | Innerleithen | 2016 |
| CM Petroleum Engineering Ltd | London | 2021 |
| Cmac Services EA Limited | Great Yarmouth | 2018 |
| CMC Mining Corporation Limited | London | 2022 |
| CMD Inspection Ltd | Isle Of Skye | 2022 |
| Cmoc UK Limited | London | 2016 |
| Cmplantservices Ltd | Ilkeston | 2023 |
| CMR Robertson Ltd | Glasgow | 2024 |
| CMS Cepcor Limited | Coalville | 1989 |
| CMS96 Ltd | Aberdeen | 2024 |
| CNG Foresight Holding Limited | London | 2020 |
| CNG Foresight Limited | London | 2020 |
| CNG Services Assets Ltd | Solihull | 2021 |
| CNG Services Group Ltd | Solihull | 2021 |
| CNG Services Ltd | Solihull | 2003 |
| Cnooc Ettrick U.K. Limited | Uxbridge | 2000 |
| Cnooc Petroleum Europe Limited | Uxbridge | 1972 |
| Cnooc Petroleum Farragon U.K. Limited | Uxbridge | 2005 |
| CNR International (U.K.) Developments Limited | Leeds | 1971 |
| CNR International (U.K.) Investments Limited | Leeds | 1977 |
| CNR International (U.K.) Limited | Leeds | 1964 |
| Cnri (G.B.) Limited | Leeds | 2009 |
| CNS (Swansea) Limited | Swansea | 2009 |
| CNS Quarry Ltd | Burford | 2024 |
| Coal Contractors Limited | Ferryhill | 1951 |
| Coals and Waste Management Ltd | Cardiff | 2025 |
| Coastal OIL and GAS Limited | Bridgend | 1997 |
| Cobasi Integrated Services Limited | London | 2020 |
| Cobban Metering Ltd | Peterhead | 2013 |
| Cobra MET UK Ltd | Manchester | 2012 |
| Cobra Resources PLC | London | 2018 |
| COC Engineering Limited | Sale | 2024 |
| Codicote Quarry Limited | Buntingford | 1993 |
| Coed Major Limited | Herefordshire | 1999 |
| Cogs Limited | Great Yarmouth | 2008 |
| Coil-Line Limited | London | 1997 |
| Coilx Ltd | London | 2026 |
| Cokebusters Limited | Flint Road | 2005 |
| Colin Douglas Consulting Limited | Edinburgh | 2021 |
| Colin Gamble Properties Limited | Derby | 1997 |
| Colin Humphrey Limited | Great Yarmouth | 2005 |
| Colin Stuart Mechanical Services Limited | Fraserburgh | 1998 |
| Collabro Limited | Aberdeen | 2003 |
| College Industrial Services Limited | Bordesley Green | 1982 |
| Collen Brothers (Quarries) Limited | Co Armagh | 1958 |
| Colombia Gold Limited | London | 2006 |
| Colorado Roughnecks Limited | Coventry | 2006 |
| Coltan Mining Limited | London | 2010 |
| Colton Quarries Lack Limited | Enniskillen | 2020 |
| Colton Quarries Limited | Belfast | 2004 |
| Columbus Energy Resources Limited | London | 2006 |
| Comag Associates Limited | Melton Constable | 2014 |
| Combat Offshore Solutions Limited | Lowestoft | 2014 |
| Comesta Mining Group Corporation Ltd | London | 2013 |
| Comet BF430 Ltd | Banff | 2013 |
| Comexpo Trading Africa Limited | London | 2023 |
| Cominco Resources (UK) Limited | Hitchin | 2010 |
| Commissioning Control and Inspection Limited | Maesteg | 1989 |
| Common Edge (PTY) Ltd | Huddersfield | 2021 |
| Commonwealth Critical Minerals Association | London | 2020 |
| Compass Minerals UK Limited | Winsford | 1991 |
| Competec UK Ltd | Sunderland | 2018 |
| Complete Conveyor Services Ltd | Belfast | 2019 |
| Complete Development Solutions Limited | Guildford | 2003 |
| Complete Well Services Limited | Lowestoft | 1996 |
| Complexdelivery Ltd | Coventry | 2026 |
| Compo (UK) Microfertilizer Ltd | Stafford | 2024 |
| Comtrack (UK) Limited | London | 2015 |
| Concave Services Limited | Kettering | 2018 |
| Concept IPR Limited | Aberdeen | 2001 |
| Concorde OIL & GAS Ltd | Aberdeen | 2018 |
| Concordia Energy Limited | Shepperton | 2024 |
| Concordia Energy Operations Limited | Shepperton | 2026 |
| Concrete Sarnau Limited | Bala | 2008 |
| Concrete Supplies Limited | Pontyclun | 2009 |
| Condor Gold Limited | London | 2005 |
| Conexpo (NI) Limited | Belfast | 1991 |
| Confined Space Services (UK) Ltd | Halifax | 2011 |
| Conlon West Africa Limited | Sleaford | 2010 |
| Conscio London Limited | London | 2020 |
| Conserve Rentals & Services Limited | Leigh | 1998 |
| Consolidated European Holdings PLC | Bakewell | 1990 |
| Consolidated Mines Ltd | St. Ives | 2025 |
| Consolidated Nickel Mines Limited | London | 2012 |
| Consortium Drilling Limited | Sheffield | 2021 |
| Construction Acuity Ltd | London | 2025 |
| Construction Recycling Technologies Group Limited | Hunstanton | 2024 |
| Contact International Inc. Limited | London | 2025 |
| Contafinvest Enterprises Limited | Brentwood | 2005 |
| Conterp Group PLC | London | 2023 |
| Continentoil Limited | London | 2007 |
| Contract OIL & GAS Inspections Ltd | Middlesbrough | 2019 |
| Contracts Consultancy Limited | London | 1981 |
| Control Cutter Limited | Aberdeen | 2019 |
| Control Distributed Trading Branch UK Ltd | Newcastle Upon Tyne | 2024 |
| Control of Work & Safety Consultants Limited | Chester | 2022 |
| Control of Work Ltd | Dingwall | 2013 |
| Controlled Bolting Solutions Ltd | Newcastle Upon Tyne | 2024 |