| Cilfeithy Ganol Limited | Newport | 2014 |
| Cilfynydd Bess Limited | Cardiff | 2022 |
| Cilfynydd Green Energy Limited | Faversham | 2021 |
| Cilfynydd Grid Services Limited | London | 2021 |
| Cilfynydd NG Limited | Cheltenham | 2023 |
| Cilgwyn Energy Limited | London | 2005 |
| Cindrigo Geothermal Limited | London | 2022 |
| CIO Smart Electric Ltd | Stevanage | 2018 |
| Cir-Lonydd Energy Limited | Stormy Down | 2026 |
| Circular Capital Limited | Devizes | 2016 |
| Cirencester Solar Farm Ltd | Warrington | 2014 |
| Citadel Estates and Utilities Limited | London | 2023 |
| Citigen (London) Limited | Nottingham | 1989 |
| City 2 City Utilities Ltd | Sunderland | 2025 |
| City Compliance-Uk Limited | Colchester | 2023 |
| City East TS Ltd | Darlington | 2014 |
| City Electrical Contractor Ltd | Cardiff | 2021 |
| City Energy Group Ltd | London | 2022 |
| City Heat GAS Services Limited | London | 2013 |
| City Intl Ltd | London | 2022 |
| City Light System Ltd | Ruislip | 2019 |
| City Renewables Ltd | Edgware | 2011 |
| Civitas Energy Ltd | Leatherhead | 2018 |
| CJ Air Installations Ltd | Bristol | 2023 |
| CJB Energy Services Limited | London | 2013 |
| CJF Travels Renewables Services Limited | Sunderland | 2026 |
| CJH Air Conditioning Ltd | Woking | 2021 |
| CJM Electrical Solutions Ltd | Gillingham | 2020 |
| CJS Heating & Air Conditioning Services Limited | Basildon | 2013 |
| CKM Electrical Limited | Coatbridge | 2014 |
| CKM Electrical Services Limited | Norwich | 2018 |
| CKZ Green Energy Ltd | Coleraine | 2015 |
| CL Armstrong Ltd | Ashington | 2023 |
| CLA GAS Limited | Durham | 2026 |
| Clach Energy Storage Limited | Edinburgh | 2026 |
| Clacton Solar Ltd | Altrincham | 2022 |
| Cladalemmanuel Electrical Contractor Limited | London | 2010 |
| Clady Haliday Wind Ltd | Cookstown | 2012 |
| Clady Hill Energy Limited | Strabane | 2013 |
| Clairmont Partners Ltd | London | 2026 |
| Clann Farm Limited | London | 2011 |
| Clapton Farm Solar Park Limited | Bristol | 2012 |
| Clara LUX Ltd | Great Yarmouth | 2017 |
| Clara Power (Developments) Ltd | Sunderland | 2026 |
| Claragh Energy Ltd | Omagh | 2020 |
| Claraghmore Wind Ltd | Portstewart | 2011 |
| Claramond Solar SPV 1 Limited | London | 2012 |
| Claremore Energy Ltd | Omagh | 2022 |
| Clarendon Solar Essex Limited | Grays | 2024 |
| Claritas Investments Ltd | London | 2021 |
| Clarke Energy Holdings Limited | Indust, Liverpool | 1999 |
| Clarke Energy Limited | Indust, Liverpool | 1977 |
| Clasa Energy Limited | Elgin | 2025 |
| Clash Gour Developments Limited | Great Missenden | 2025 |
| Clash Gour Wind Farm Limited | Great Missenden | 2025 |
| Clash Gour Windfarm Holdings Limited | Edinburgh | 2018 |
| Clashindarroch II Wind Farm Limited | Edinburgh | 2024 |
| Clashindarroch Wind Farm Limited | London | 2005 |
| Clashindarroch Windfarm Extension Limited | Ringwood | 2020 |
| Class-A Green Energy UK Limited | London | 2022 |
| Classic Wheels Limited | Gosport | 1993 |
| Clatto Wind Ltd | Dundee | 2022 |
| Claverhouse Generation Limited | Glasgow | 2018 |
| Clavering Electrical Services Limited | Saffron Walden | 2013 |
| Clawdd DDU Farm Limited | London | 2013 |
| Clawddcam Solar Park Ltd | Romsey | 2017 |
| Clay Lane Solar Limited | Liverpool | 2019 |
| Clayhill Solar Limited | Swindon | 2013 |
| Claylands Farm Solar Limited | Liverpool | 2023 |
| Claymore Electrical Services Limited | Inverurie | 2007 |
| Clays Farm Solar Limited | Edinburgh | 2021 |
| Clayton Renewables Ltd | East Lothian | 2014 |
| Claywell Ltd | Southwell | 2017 |
| Clean Air Conditioning Installation Limited | Sutton | 2017 |
| Clean Battery Power UK 1 Limited | London | 2022 |
| Clean Battery Power UK 2 Limited | London | 2022 |
| Clean Earth Energy Investments Limited | Wadebridge | 2011 |
| Clean Earth Ground Mount PV Limited | Wadebridge | 2015 |
| Clean Earth Solar Investments Ltd | Wadebridge | 2015 |
| Clean Earth Wind Turbines Ltd | Wadebridge | 2014 |
| Clean Electricity Limited | London | 2011 |
| Clean Energy CAP Ltd | London | 2025 |
| Clean Energy Gurus Limited | Barnsley | 2026 |
| Clean Energy Projects Ltd | Christow | 2020 |
| Clean Invest Africa PLC | London | 2017 |
| Clean Power Generation Limited | Norwich | 2011 |
| Clean Power UK Wind Limited | London | 2024 |
| Clean Solar Power UK 1 Ltd | London | 2022 |
| Cleancurve Connect Ltd | Birmingham | 2022 |
| Cleanfleet Energy Group Limited | Northampton | 2026 |
| Clear Power Generation Limited | Coventry | 2026 |
| Clear Volt Limited | Nether Alderley | 2024 |
| Clearsky Power Limited | Coventry | 2025 |
| Clearstone Energy Limited | London | 2016 |
| Clearstone ITA Limited | London | 2024 |
| Clearstone One Limited | London | 2017 |
| Clearstone Services Limited | London | 2016 |
| Clearwell Limited | Michaelston-Y-Fedw | 2012 |
| Cleat Hill Energy Limited | Swadlincote | 2014 |
| Cleator Battery Storage Limited | London | 2016 |