| Cornish Wind Ltd | London | 2013 |
| Cornwall Energy Storage Ltd | Torpoint | 2022 |
| Cornwall Power (Polmaugan) Limited | London | 2011 |
| Cornwall Renewable Developments Limited | Launceston | 2011 |
| Cornwell Solar Farm Limited | London | 2019 |
| Coro Energy PLC | Leeds | 2016 |
| Corona Energy Retail 1 Limited | Watford | 1996 |
| Corona Energy Retail 2 Limited | Edinburgh | 1992 |
| Corona Energy Retail 3 Limited | Watford | 1992 |
| Corona Energy Retail 4 Limited | Watford | 1993 |
| Corpgreen Ltd | Altrincham | 2025 |
| Corporate Energy and Renewables Ltd | Glasgow | 2017 |
| Corporate Energy Ltd | Glasgow | 2025 |
| Corporate Power Limited | Romford | 2011 |
| Corpower Limited | Salford Quays | 2016 |
| Corr Limited | Antrim | 2015 |
| Correctair Solutions (Scotland) Ltd | Glasgow | 2019 |
| Corrennie Associates Limited | London | 2015 |
| Corrfield Limited | Carmarthen | 2014 |
| Corriegarth Wind Energy Limited | Edinburgh | 2014 |
| Corrimony Energy Limited | Inverness | 2009 |
| Corrinare Wind Limited | Belfast | 2016 |
| Corringham Power Limited | Salford Quays | 2023 |
| Corsham Solar Farm Limited | London | 2022 |
| Corshellach Green Energy Storage Ltd | London | 2022 |
| Cortes Renewables Ltd | Fraserburgh | 2012 |
| Cosens and Company,Limited | Southampton | 1876 |
| Costa Head Wind Farm Limited | Glasgow | 2010 |
| Costessey Energy Limited | Northampton | 1997 |
| Costlook Company Limited | Aldershot | 1993 |
| Costock Solar Farm Limited | London | 2020 |
| COT Energy Ltd | Manchester | 2021 |
| COT Projects Ltd. | Newark | 2011 |
| Cotes Heath Solar Limited | London | 2024 |
| Cotesbach Energy Limited | London | 1999 |
| Cotswold Cleantech Energy Ltd | Cheltenham | 2026 |
| Cotswold Electrical Limited | Gravesend | 2019 |
| Cotswold Refrigeration Ltd | Gloucester | 2023 |
| Cottom to Ryhall Energy Limited | London | 2023 |
| Cotton Farm Wind Farm Limited | London | 2011 |
| Coubar Solar Limited | London | 2020 |
| Cougar Limited | Southampton | 1980 |
| Coulags Hydro Ltd | Perth | 2014 |
| Coulomb Energy Supply Limited | Thornaby | 2011 |
| Cound Park District Heating Ltd | Cound | 2014 |
| Counterflow Energy Services Ltd | Glasgow | 2020 |
| Country GAS Limited | Driffield | 2003 |
| Country Utilities Ltd | Corby | 2015 |
| Countryside Renewables (Forest Heath) Ltd | London | 2011 |
| Countryside Renewables Ltd | Chislehurst | 2013 |
| Coupar Limited | London | 2019 |
| Coupar Two Limited | London | 2021 |
| Cour Wind Farm (Scotland ) Limited | Edinburgh | 2014 |
| Court Colman Solar Limited | Faversham | 2013 |
| Court Farm Solar Farm Limited | Gatwick | 2016 |
| Court Road Energy Limited | Exeter | 2024 |
| Court Solarfarm Limited | London | 2014 |
| Courts Air Conditioning Limited | Bexley | 2012 |
| Cousland Park Ltd | Tranent | 2011 |
| Covanta Holding UK Limited | London | 2017 |
| Covarrubia Energy GAS & UK Ltd | Glasgow | 2025 |
| Covarrubia Energy GAS UK Limited | Manchester | 2026 |
| Covarrubia Energy UK Ltd | Glasgow | 2019 |
| Coven Solar Holdco Limited | Reading | 2024 |
| Coven Solar Limited | Reading | 2024 |
| Coventry District Energy Company Limited | London | 2012 |
| Coventry Energy Storage Limited | London | 2021 |
| Coventry Power Limited | Wolverhampton | 2022 |
| Covesea Renewables Limited | Shepton Mallet | 2025 |
| COW Marsh Wind Farm Limited | Harrogate | 2024 |
| COW Pasture Energy Limited | Telford | 2014 |
| Cowdown Green Energy Limited | London | 2022 |
| Cowley Baldon Green Limited | Preston | 2020 |
| Cowley Battery Storage Limited | London | 2025 |
| Cowling Energy Limited | Goole | 2012 |
| Cozy Climate Systems Ltd | Redditch | 2025 |
| CP Energy Centre Ltd | Liverpool | 2022 |
| CP Project 1 Ltd | Redditch | 2025 |
| CP Project Holdings Ltd | Redditch | 2024 |
| CP Utilities Limited | Tilbury | 2024 |
| CPD Powerline Ltd | Armagh | 2005 |
| CPH Air Ltd | Huddersfield | 2020 |
| CPL Air Conditioning Limited | Derby | 2026 |
| CPL81 Ltd | London | 2019 |
| CPM Renewables Limited | Romford | 2025 |
| CPN Electrical Limited | Rochester | 2023 |
| CPS Solutions Group Limited | Barnet | 2020 |
| Cracka Longi Limited | Stromness | 2017 |
| Craft-E.Com Ltd. | Church Stretton | 1998 |
| Cragg Power Ltd | Bradford | 2015 |
| Craig Energy Supply Limited | Borehamwood | 2025 |
| Craig Hydro Limited | Dunblane | 2020 |
| Craig NAB Wind Energy Hub Limited | Bristol | 2022 |
| Craig North Limited | London | 2025 |
| Craig Wind Farm Limited | London | 2005 |
| Craig Y Geifr Wind Energy Hub Limited | Bristol | 2021 |
| Craigannet WT Limited | Glasgow | 2016 |
| Craigdarroch Wind Farm Ltd | Gloucestershire | 2025 |
| Craigdoo Power Limited | Ballymena | 2015 |
| Craiglour Limited | Thornhill | 2015 |