| Community Energy Together Limited | London | 2021 |
| Community Energy Twemlows 2 C.I.C. | Shrewsbury | 2015 |
| Community Energy Twemlows C.I.C. | Shrewsbury | 2015 |
| Community EV Charging Limited | London | 2024 |
| Community Geothermal CIC | Collaton St Mary | 2025 |
| Community Infrastructure Scilly C.I.C. | Isles Of Scilly | 2023 |
| Community of Stãll CIC | Leeds | 2024 |
| Community Wind Power (Securities) Limited | Edinburgh | 2019 |
| Community Windpower Limited | Warrington | 2002 |
| Comnergy Ltd | London | 2025 |
| Comoro Energy Limited | London | 2017 |
| Compact Cooling Limited | Bromley | 2002 |
| Compare Business Utilities Ltd | Welford Upon Avon | 2020 |
| Compare to Share CIC | Newcastle Upon Tyne | 2020 |
| Compass Electrical Installations Limited | East Morton | 2017 |
| Compass Power Solutions Ltd | Liverpool | 2026 |
| Compex Hazardous Inspection Ltd | Crook | 2014 |
| Complete Air Conditioning Services Ltd | Braintree | 2025 |
| Complete Biomass Solutions Ltd | Dudley | 2021 |
| Complete Metering Services Ltd | Bolton | 2018 |
| Complete Planned Maintenance Limited | Cambridge | 1996 |
| Complete Solar Energy Solutions UK Ltd | London | 2019 |
| Complete Temperature Control Ltd | Basingstoke | 2012 |
| Complete Utility Connections Ltd | York | 2025 |
| Complete Utility Services Ltd | Doncaster | 2022 |
| Completeair (UK) Ltd | Gainsborough | 2020 |
| Comprehensive Property Maintenance Ltd | Watford | 2019 |
| Compressed Air Systems Limited | Newport | 2015 |
| Compressorman Ltd | London | 2023 |
| Compton Solar Limited | Morriston | 2011 |
| Comsol Energy Limited | Bellshill | 2015 |
| CON Air Heating, Ventilation & Cooling Limited | Portsmouth | 2023 |
| CON Air Installations Ltd | Southampton | 2011 |
| CON Air Northern Limited | Wakefield | 2022 |
| Con-Fused Electrical Limited | Leyland | 2010 |
| Concept Environments Limited | Hartley Wintney | 2008 |
| Condair Limited | Littlehampton | 1982 |
| Condate Biogas Limited | London | 2014 |
| Condensed Cooling Ltd | Manchester | 2018 |
| Condition Environment Air Services Limited | London | 2019 |
| Condition Environment Services Limited | London | 1993 |
| Conditioned Environment Regions Limited | Glasgow | 2021 |
| Conditioner KOP Limited | Great Harwood | 2025 |
| Conduct Electrical Services Ltd | Maidstone | 2010 |
| Conerick Power Limited | Enniskillen | 2013 |
| Conesby Grid Limited | Norwich | 2024 |
| Conesby Solar Farm Limited | Norwich | 2018 |
| Conharlil Limited | Glasgow | 2015 |
| Coniferous Ltd | Waltham Cross | 2024 |
| Coniston Energy Limited | Bicester | 2018 |
| Coniston Hydro Electric Power Limited | Coniston | 2003 |
| Connahs Quay Energy Limited | London | 2023 |
| Connaught Wind Limited | Antrim | 2010 |
| Connectid 1 Limited | Guildford | 2023 |
| Connectid 2 Limited | Guildford | 2023 |
| Connectid 3 Limited | Guildford | 2023 |
| Connectid 9 Limited | Guildford | 2023 |
| Connection Solutions Limited | Ellesmere Port | 2012 |
| Connects Services Ltd | Croydon | 2021 |
| Connell Ventures Limited | Auchterarder | 2012 |
| Connex Renewables Ltd | Newry | 2011 |
| Connick Renewable Energy Ltd | Launceston | 2012 |
| Connon Bridge Energy Limited | London | 1999 |
| Connor's Heating Solutions Ltd | Shildon | 2018 |
| Conrad (2022) Limited | Abingdon | 2020 |
| Conrad (Alcoa) Limited | Abingdon | 2020 |
| Conrad (Ampthill) Limited | Abingdon | 2017 |
| Conrad (Arncott) Limited | Abingdon | 2018 |
| Conrad (Barton Hill) Limited | Abingdon | 2020 |
| Conrad (Beighton) Limited | Abingdon | 2021 |
| Conrad (Belton Farm) Limited | Abingdon | 2025 |
| Conrad (Benfleet) Limited | Abingdon | 2016 |
| Conrad (Billericay) Limited | Abingdon | 2020 |
| Conrad (Bilsham) Limited | Abingdon | 2022 |
| Conrad (Bilston) Limited | Abingdon | 2018 |
| Conrad (Bishopstoke) Limited | Abingdon | 2022 |
| Conrad (Bispham) Limited | Abingdon | 2016 |
| Conrad (Blackpool) Limited | Abingdon | 2020 |
| Conrad (Bletchley) Limited | Abingdon | 2015 |
| Conrad (Bolton a) Limited | Abingdon | 2023 |
| Conrad (Bolton B) Limited | Abingdon | 2023 |
| Conrad (Bootle) Holdco Limited | Abingdon | 2013 |
| Conrad (Bootle) Limited | Abingdon | 2007 |
| Conrad (Bridgwater) Limited | Abingdon | 2021 |
| Conrad (Caernarfon) Limited | Abingdon | 2017 |
| Conrad (Calne) Limited | Abingdon | 2017 |
| Conrad (Castle Bromwich) Limited | Abingdon | 2020 |
| Conrad (Chatterley) Limited | Abingdon | 2016 |
| Conrad (Cleveleys) Limited | Abingdon | 2017 |
| Conrad (Crewkerne) Limited | Abingdon | 2021 |
| Conrad (CV1) Limited | Abingdon | 2025 |
| Conrad (Dowlais) Limited | Abingdon | 2020 |
| Conrad (Down Barn Farm) Limited | Abingdon | 2020 |
| Conrad (Drove) Limited | Abingdon | 2020 |
| Conrad (Dunkeswell) Limited | Abingdon | 2017 |
| Conrad (Emerald) Limited | Abingdon | 2020 |
| Conrad (EYE) Limited | Abingdon | 2022 |
| Conrad (Felixstowe) Limited | Abingdon | 2017 |
| Conrad (Fleetwood) Limited | Abingdon | 2015 |
| Conrad (Flint) Limited | Abingdon | 2017 |