| Custom Climates Limited | Lancing | 2011 |
| Custom Cooling Solutions Ltd | Cardiff | 2022 |
| Customair Limited | Hornchurch | 2026 |
| Cutlers Solar Farm Limited | London | 2020 |
| Cuxton Energy Storage Limited | London | 2020 |
| Cvac Internatonal Limited | London | 2025 |
| Cvcwd Limited | Macclesfield | 2023 |
| CVN Ventures Ltd | London | 2025 |
| CW Systems Ltd | Ruislip, | 2025 |
| CW UK Trading Ltd | Chorley | 2019 |
| CWE Airdrie Limited | Godalming | 2014 |
| CWE Belhus Limited | Godalming | 2016 |
| CWE Donaldson Limited | Godalming | 2016 |
| CWE DS Limited | Godalming | 2013 |
| CWE Endurance Limited | Godalming | 2011 |
| CWE Endure Limited | Godalming | 2014 |
| CWE Gardrum Limited | Godalming | 2014 |
| CWE Jacobshall Limited | Godalming | 2014 |
| CWE Lochend Limited | Godalming | 2021 |
| CWE Meikle Float Limited | Godalming | 2013 |
| CWE Northwind Limited | Godalming | 2011 |
| CWE Norwin Limited | Godalming | 2012 |
| CWE NW3 Limited | Godalming | 2014 |
| CWE RTW Limited | Godalming | 2014 |
| CWE Struan Limited | Godalming | 2013 |
| CWE Turbines Limited | Godalming | 2020 |
| CWE V27 Limited | Godalming | 2015 |
| CWE Vestas Limited | Godalming | 2014 |
| CWE WTN Limited | Godalming | 2014 |
| CWE WUK Limited | Godalming | 2011 |
| CWE X Limited | Godalming | 2015 |
| CWL Energy Limited | Edinburgh | 2017 |
| CWM Cadian Hydro Limited | Machynlleth | 2018 |
| CWM Cloch Hydro Limited | Caernarfon | 2013 |
| CWM Cloch MCJ Hydro Ltd | Caernarfon | 2021 |
| CWM Ifor Solar Limited | London | 2026 |
| Cwmni Hydro Dwrnudon CYF | Mold | 2015 |
| Cwmni Singrug CYF | Y Bala | 2016 |
| CWP Energy Limited | Edinburgh | 2024 |
| CWS Energy Co Limited | Blairgowrie | 2016 |
| CWS Hold Co Limited | Milton Keynes | 2015 |
| Cycledelik Limited | London | 2015 |
| Cyflymen Hydro Ltd | Llangollen | 2010 |
| Cygnet Renewables Limited | Ascot | 2022 |
| Cygnus Marine Holdings Limited | Ivybridge | 1993 |
| Cylinder Sales Ltd | Warrington | 2021 |
| Cymric Limited | Ludlow | 2002 |
| Cynon Power Limited | Bristol | 2013 |
| CYP Solar Limited | London | 2019 |
| Cyrus Electrical Limited | Middlesbrough | 2023 |
| D & C Facilities Ltd | East-Dulwich | 2013 |
| D & G Air Conditioning (Services) Ltd | Tredegar | 2015 |
| D & L J Wilson Farming Limited | Lincoln | 2019 |
| D & S Chlorination Services Limited | Brentwood | 2016 |
| D C Brazendale Heating Ltd | Northwich | 2017 |
| D Electric Services Ltd | Basildon | 2025 |
| D Griffiths Electrical Services Limited | Tamworth | 2009 |
| D Illingworth Utilities Limited | Benfleet | 2025 |
| D J Mechanical Ltd | Barnsley | 2025 |
| D J Orrin Commissioning Services Limited | Chelmsford | 2013 |
| D Jones Utilities Limited | Welwyn Garden City | 2024 |
| D N Bepswa Limited | Corby | 2025 |
| D P Electrical (NW) Limited | Blackpool | 2018 |
| D P Refrigeration Limited | Bradford | 2012 |
| D Royale Ltd | Hounslow | 2013 |
| D T Electrical SW Limited | Taunton | 2014 |
| D Taylor Air Conditioning Ltd | Langley Mill | 2019 |
| D W Allen Electrical Ltd | Ferndown | 2014 |
| D W V Utility Solutions Ltd | Stockport | 2011 |
| D Ward Energy Ltd | Middlesbrough | 2023 |
| D Ward Ltd | Middlesbrough | 2015 |
| D Wright Utilities Limited | Ashby-De-La-Zouch | 2019 |
| D&A Air Conditioning Ltd | St. Neots | 2011 |
| D&B Electrical Contractors Ltd | Winchester | 2013 |
| D&G Air Conditioning Ltd | Antrim | 2013 |
| D&O South Coast Solar Ltd | Poole | 2023 |
| D&P Electrical (Abersoch) Limited | Pwllheli | 2011 |
| D-Energi Trading Limited | Manchester | 2017 |
| D-Tech Security Services Limited | Leeds | 2005 |
| D-Volts Electrical Services Ltd | Birmingham | 2022 |
| D. Wade Electrical Contractors Limited | Southwick | 2008 |
| D.B. Green Limited | Brooks Road Lewes | 1974 |
| D.B.L.F. Electrical Limited | Hensford | 2013 |
| D.Booth Electrical Ltd | Ruislip | 2018 |
| D.C.S (EK) Ltd | Glasgow | 2024 |
| D.D.E Services Ltd | Heathfield | 2005 |
| D.E.A.R. Renewables Limited | Stonehaven | 2014 |
| D.G.Gas Services Limited | Leicestershire | 2003 |
| D.H Electrical Engineering Services Ltd | Sheffield | 2015 |
| D.Heating Care Ltd | London | 2022 |
| D.J. Allen Electrical Contractors Limited | Northants | 2003 |
| D.L.P GAS Systems Ltd | Redditch | 2011 |
| D.M. Hardy Electrical Services Ltd | Chatham | 2008 |
| D.O.S Electrical Limited | Ilford | 2015 |
| D.S.L. GAS FIX Ltd | Hebburn | 2016 |
| D1 Air-Con Ltd | London | 2022 |
| D1 Mechanical Services Ltd | Portsmouth | 2019 |
| D17 Air-Conditioning Ltd | Hockley | 2026 |
| D200 Energy Limited | Nottingham | 2014 |
| D2M Solutions Limited | Southampton | 2002 |