| CNS Energy Limited | Kings Langley | 2024 |
| CO2SENSE Pantycelyn Limited | London | 2015 |
| Coalburn Energy Limited | London | 2024 |
| Coalfire Cottage Ltd | Ammanford | 2026 |
| Coastal Breeze Air Conditioning Ltd | Christchurch | 2026 |
| Coastal Energy Limited | Exeter | 2017 |
| Coastline Air Conditioning Services Limited | Littlehampton | 2026 |
| Coastline Cooling Services Ltd | Ferndown | 2023 |
| Coating Investments Ltd | Bury St. Edmunds | 2025 |
| Cobalt Energy PL Ltd | Croydon | 2019 |
| Cobalt Power Limited | London | 2010 |
| Cobana Energy Ltd | London | 2015 |
| Cobana Limited | London | 2021 |
| Cobblestone Energy Ltd | London | 2016 |
| Cobham Road Solar Farm Limited | Exeter | 2023 |
| Cobra Castle Solar Farm Limited | Bristol | 2022 |
| Cobra Energy Ltd | Portsmouth | 2018 |
| Cobra T&D Limited | Glasgow | 2025 |
| Cobwood Solar Farm Limited | London | 2022 |
| Cochrane Shipbuilders Limited | Chester Le Street | 1975 |
| Cockburn Energy Ltd | Duns | 2013 |
| Cockenzie Storage Limited | London | 2017 |
| Cockerham Green Energy Limited | Lancaster | 2010 |
| Code Orangery Ltd | Edinburgh | 2023 |
| Codford Biogas Limited | Norwich | 2012 |
| Codlaw Renewable Energy Limited | Hexham | 2013 |
| Codrington Energy Centre Ltd | Norwich | 2021 |
| Coed Adam Energy Ltd | Kidwelly | 2014 |
| Coedcae Farm Energy Limited | Aberdare | 2025 |
| Coega Biomass Processing Limited | Romsey | 2020 |
| Coega Hydrogen Limited | Romsey | 2020 |
| Coega Solar Park Limited | Romsey | 2020 |
| Cogeneration Holdings Limited | London | 2009 |
| Coglan Airconditioning Services Limited | Basingstoke | 2002 |
| Coigach Community C.I.C. | Manchester | 2015 |
| Coille Beith Wind Farm Limited | London | 2021 |
| Coille-Dhealain Ltd | Auchterarder | 2017 |
| Coiltie Hydro Limited | Perth | 2011 |
| Coire Glas Hydro Pumped Storage Limited | Perth | 2017 |
| Colada Energy Ltd | London | 2024 |
| Colbon GOC (NI) Limited | Dungannon | 2009 |
| Colbrans Solar ESS Limited | Cheltenham | 2024 |
| Colburn Grange Limited | Catterick Garrison | 2017 |
| Colchen Solar Limited | Macclesfield | 2011 |
| Cold FIX Ltd | London | 2025 |
| Cold Northcott Windfarm Limited | Camelford | 1991 |
| Cold Pro Ltd | West Yorkshire | 2021 |
| Cold Services Limited | London | 2024 |
| Cold Station Ltd | London | 2023 |
| Colden Water Power Company Limited | Hebden Bridge | 2014 |
| Coldham Windfarm Limited | | 2004 |
| Coldharbour Farm Solar Park Limited | Shepton Mallet | 2015 |
| Coldpress Group Limited | Winchester | 2014 |
| Coldroom Solution Ltd | Bordon | 2024 |
| Coldstream Wind Community Limited | Carluke | 2011 |
| Cole Climate Solutions Limited | Hatfield | 2024 |
| Coleby Grange Energy Reserve Ltd | Faversham | 2025 |
| Coleman UK Group Limited | Hornchurch | 2017 |
| Colin Burns Electrical Services Limited | Paisley | 2014 |
| Coll Power Limited | Malton | 2015 |
| Collace Solar Limited | Edinburgh | 2021 |
| Collapse Breeze Air Ltd | Southend-On-Sea | 2025 |
| College Farm Hydro Limited | Ballymena | 2015 |
| Collingwood Street Solar Limited | London | 2018 |
| Collins Electrical Services Limited | Renfrew | 2006 |
| Colne Mechanical Services Limited | Colchester | 2003 |
| Colsterworth Energy Limited | London | 1998 |
| Colwydr CYF | Caernarfon | 2018 |
| Combined Energy Limited | Norwich | 2016 |
| Comfort Air AC Ltd | Leicester | 2025 |
| Comfort Air Solutions Ltd | West Malling | 2023 |
| Comfort Climate Services Ltd | Southhall | 2025 |
| Comfort Cooling Installations Limited | Hertfordshire | 2007 |
| Comfort Cooling West Midlands Ltd | Worcester | 2025 |
| Comfort Life Home Appliances Co., Limited | Beckenham | 2014 |
| Commercial and SME Management Ltd | Washington | 2022 |
| Commercial Catering & Heating Engineer Specialist Ltd | London | 2023 |
| Commercial Community Energy Scheme Stoke Limited | Holborn | 2018 |
| Commercial Ducting Ltd | Maidstone | 2016 |
| Commercial Energy Group Limited | Fairford | 2019 |
| Commercial Energy Switch Limited | Solihull | 2024 |
| Commercial Pipework Solutions Ltd | Scarborough | 2017 |
| Commercial Projects International Limited | London | 2025 |
| Commercial Synergy Ltd | Clitheroe | 2024 |
| Commercial Trade Service Electrical Ltd | London | 2014 |
| Commercial Utility Billing Advisor (Cuba) Ltd | Bagillt | 2012 |
| Commercial Utility Consultants Limited | Bradford | 2009 |
| Commercial Utility Support Service Ltd | Manchester | 2019 |
| Commercis Engineering Ltd | London | 2018 |
| Commissioning & Maintenance Services Limited | Stockport | 1989 |
| Commodities Brain Limited | Weybridge | 2023 |
| Common Barn Wind Farm Ltd | Abingdon | 2012 |
| Common Farm Solar Farm Limited | London | 2015 |
| Common Farm Solar Park Limited | Houghton-Le-Spring | 2021 |
| Commotex Limited | Hinckley | 2017 |
| Community Energy Lower Basset Down C.I.C. | Saffron Walden | 2015 |
| Community Energy Partnerships (UK) Limited | Castleford | 2018 |
| Community Energy Scheme Stoke Limited | Ware | 2018 |
| Community Energy Scheme UK Limited | Ware | 2017 |
| Community Energy Scotland Trading Limited | Inverness | 2008 |