| Clemson Electrical Services (Northwest) Ltd | Burnley | 2006 |
| Clenergy (Project 4) Ltd | Preston | 2015 |
| Cleughead Limited | Elgin | 2012 |
| Cleve Hill Solar Park Limited | London | 2014 |
| Clever Power UK Holdings Ltd | Wicken | 2026 |
| Cleveside Ggrs Ltd | London | 2026 |
| CLG Air Conditioning Services Limited | Kettering | 2002 |
| CLH Solar & Renewable Energy Limited | Cambridge | 2023 |
| Click Energy Limited | London | 2017 |
| Click to Switch Ltd | Swindon | 2022 |
| Cliffdon Ltd | Cookstown | 2006 |
| Cliffe Storage Limited | London | 2022 |
| Clifton Agricultural Waste Plant Holdings Limited | London | 2026 |
| Clifton Agricultural Waste Plant Limited | London | 2026 |
| Clifton Biofuel Plant Limited | London | 2026 |
| Clifton Solar Ltd. | Stonehouse | 2018 |
| Clifton Solar Project Limited | London | 2022 |
| Clima Hogar Kazuki Limited | London | 2019 |
| Clima Works Hvac Ltd | Glasgow | 2024 |
| Clima-Tech Hvac Ltd | Ipswich | 2019 |
| Climac Ltd | Cardiff | 2022 |
| Climacool Air Conditioning Limited | Birmingham | 2020 |
| Climafleet UK Ltd | Sheffield | 2018 |
| Climatair Limited | Palmers Green | 2023 |
| Climate Advisory Solutions Limited | London | 2025 |
| Climate Care Installations Ltd | Walsall | 2023 |
| Climate Care Services UK Limited | Lichfield | 2015 |
| Climate Cooling & Heating Ltd | Waterlooville | 2025 |
| Climate ER Ltd | Bath | 2019 |
| Climate Index Network Ltd. | London | 2025 |
| Climate Innovation District 312 Utilities Ltd | Bradford | 2017 |
| Climate Innovation District Utilities 216 Ltd | Bradford | 2022 |
| Climate Innovation District Utilities 247 Ltd | Bradford | 2020 |
| Climate Installations UK Ltd | Chesterfield | 2014 |
| Climate Project Solutions Limited | Leigh On Sea | 2017 |
| Climate Services 4 U Limited | Hemel Hempstead | 2014 |
| Climate Solutions Refrigeration and Air Conditioning Limited | Cardiff | 2021 |
| Climatebow Ltd | London | 2025 |
| Climatech Air Conditioning Limited | Farnborough | 2002 |
| Climatech Cooling and Heating Limited | Rotherham | 2022 |
| Climatic Controls Ltd | Stourport-On-Severn | 2024 |
| Climatise Hvac Services Ltd | Chorley | 2018 |
| Climatize Air Conditioning Limited | Blackpool | 2018 |
| Climatize Limited | Loughborough | 2020 |
| Climetech Ltd | Northampton | 2025 |
| Climexa AI Ltd | Uxbridge | 2025 |
| Climpy WT Limited | Glasgow | 2011 |
| Clinking Cauldron Wind Farm Limited | London | 2024 |
| Clintec Ltd. | Sheffield | 2015 |
| Clinton Renewables Limited | Exeter | 2014 |
| Clipper Electrical Limited | Inverness | 2017 |
| Clique Energy Ltd | Bolton | 2014 |
| Clive Collett Electrical Limited | Newbury | 2002 |
| Clixby Biogas Limited | Market Rasen | 2012 |
| CLM Airconditioning Ltd | Cannock | 2018 |
| Clochnahill Limited | Stonehaven | 2008 |
| Clockwork Orange Ltd | South Shields | 2025 |
| Cloford Common Farm Limited | London | 2012 |
| Clogau Renewable Energy Limited | Matlock Bath | 1985 |
| Clogher Valley Farm Energy Ltd | Fivemiletown | 2011 |
| Clogwyn Hydro Limited | Caernarfon | 2014 |
| Cloich Windfarm Limited | Sunderland | 2010 |
| Cloonty Windfarm Limited | Belfast | 2010 |
| Clos YR Efail Energy Storage Park Limited | London | 2024 |
| Close UP Inspection Ltd | Westhill | 2014 |
| Closecooper Energy Ltd | Darlington | 2025 |
| Closecooper Utilities Consultancy Ltd | Darlington | 2025 |
| Cloud Industry Ltd | London | 2023 |
| CLOUD9 Power Limited | Portsmouth | 2026 |
| Cloudnine Limited | Ilford | 2016 |
| Clover Hill Windfarm Limited | Newry | 2018 |
| Clover Solar Ltd | Preston | 2025 |
| Cloyfin Energy Ltd | Cookstown | 2011 |
| CLP Envirogas Limited | London | 1999 |
| Clpe Projects 1 Limited | London | 1997 |
| Clpe ROC - 1 Limited | London | 2003 |
| Clpe ROC - 2 Limited | London | 2004 |
| Clpe ROC - 3 Limited | London | 2004 |
| CLR Energy Limited | Marske-By-The-Sea | 2024 |
| Clune Energy Limited | Glasgow | 2022 |
| Cluntagh Renewables Ltd | Crossgar | 2014 |
| Clyde Power and Renewables Limited | Greenock | 2019 |
| Clyde South Energy Park Limited | Glasgow | 2025 |
| Clyde Windfarm (Scotland) Limited | Perth | 2005 |
| Clydeview Energy Park Limited | Glasgow | 2024 |
| Clyne Power Limited | Bristol | 2014 |
| Clynemilton Energy Ltd | Brora | 2013 |
| Clywynni Cyfyngedig | Dolgellau | 2015 |
| CM Cabling Limited | Colchester | 2017 |
| CM Natural Energy Ltd | Ashbourne | 2012 |
| CM Wind Ltd | Pontefract | 2022 |
| CMC Electrical Installers Limited | Alcester | 2009 |
| CMD Utilities Ltd | Paisley | 2019 |
| CME Utilities Ltd | Lichfield | 2023 |
| CML Electrical Ltd | New Malden | 2021 |
| CNG Energy Limited | London | 1999 |
| CNK (Coll) Limited | Perth | 2014 |
| Cnoc Buidhe Wind Energy Hub Limited | Bristol | 2021 |
| Cnoc Cailliche Energy Community Interest Company | Edinburgh | 2016 |
| Cnooc U.K. Marketing Ltd | London | 2019 |