| Crofton Carbon Ltd | Leeds | 2019 |
| Crogga (UK) Ltd | Altrincham | 2018 |
| Cromar Consultants Ltd | Edinburgh | 2016 |
| Crombie Enterprises Ltd | Berkshire | 1999 |
| Crome Engineering Limited | Stonehaven | 2013 |
| Cromwell Wood Estate Company Limited | Wakefield | 2002 |
| Crondall Energy Consultants Ltd | Surrey | 2000 |
| Crondall Energy Holdings Ltd | Farnham | 2024 |
| Crondall Energy Ltd | Farnham | 2018 |
| Cronin Inspection Services Ltd | Stowmarket | 2020 |
| Cross Keys Energy Limited | Guildford | 2017 |
| Cross-Services Ltd | Aberdeen | 2013 |
| Crossgills Sand Company Limited | Clitheroe | 1975 |
| Crow Wood Enterprises Limited | Warrington | 2025 |
| Crowdafrik Limited | London | 2019 |
| Crown Aggregates Ltd | Nuneaton | 2017 |
| Crown Global Energy Ltd | London | 2023 |
| Crownstep Limited | London | 2019 |
| CRR Services (Scotland) Limited | Cupar | 2011 |
| Crucis Australis Mining Ltd | Hereford | 2023 |
| Crude Engineering Limited | London | 2008 |
| Cruickshank Support Services Ltd | Peterhead | 2023 |
| Crushing Ltd | London | 2025 |
| Crushinput Limited | South Lanarkshire | 2003 |
| Crypto Rigs Ltd | Margate | 2021 |
| Crystal Classics Fine Minerals Limited | Yeovil | 2011 |
| CS Access Solutions Limited | Normanton | 2007 |
| CS Mechanical Connections Ltd | Ashington | 2021 |
| CS Pipeline Engineering Services Ltd | Aberdeenshire | 2007 |
| CSL Plumbing and Heating Limited | Castleford | 2025 |
| Csqms Ltd | Preston | 2013 |
| CSW Matco Consultants Ltd | Aberdeen | 2024 |
| CT MAC Ltd | Dingwall | 2013 |
| CT24-OFFSHORE Ltd | Dundee | 2026 |
| CTA Transandina Limited | London | 2017 |
| CTC GEO Ltd | Stanley | 2005 |
| Ctechoffshore Limited | Glasgow | 2017 |
| CTJ Moore Ltd | Alnwick | 2025 |
| CTL Minerals Limited | London | 2026 |
| CTM Energy Ltd | Woking | 2026 |
| CTS Geophysics Ltd | West Sussex, Crawley | 2018 |
| Ctylr-Company Ltd | Dundee | 2019 |
| Cuadrilla Balcombe Limited | Manchester | 2009 |
| Cuadrilla Bowland Limited | Manchester | 2012 |
| Cuadrilla Elswick (NO.2) Limited | Manchester | 2010 |
| Cuadrilla Elswick Limited | Manchester | 2009 |
| Cuadrilla Gainsborough Limited | Manchester | 2016 |
| Cuadrilla North Cleveland Limited | Manchester | 2016 |
| Cuadrilla Resources Limited | Manchester | 2008 |
| Cuadrilla South Cleveland Limited | Manchester | 2016 |
| Cuadrilla Weald Limited | Manchester | 2009 |
| Cuadrilla Well Services Limited | Manchester | 2009 |
| Cuffe Harrell and Associates Limited | Surrey | 1994 |
| Cumbria Quarrying Services Limited | Penrith | 2005 |
| Cumbrian Stone Limited | Penrith | 2006 |
| Cutting Underwater Technologies Ltd. | Aberdeen | 1999 |
| CW Project Controls Ltd | Glasgow | 2016 |
| Cware Crug-Yr-Eryr CYF | Ceredigion | 2003 |
| Cware Pantgwyn Quarry Limited | Crymych | 1999 |
| CWJ Projects Ltd | Wigan | 2023 |
| CWM Bargoed Mining Limited | Bargoed | 2002 |
| Cyan Commodities Limited | Llantarnam Industrial Park | 2021 |
| Cyan Subsea UK Ltd. | Aberdeen | 1994 |
| Cycletime Limited | Chichester | 2001 |
| Cyclone Metals Limited | London | 2026 |
| Cygnas Solutions Limited | Aberdeen | 2015 |
| Cymru Quarry Group Limited | Haverfordwest | 2001 |
| D & G Well Services Ltd | Lossiemouth | 2025 |
| D & I Rutherford Limited | Tain | 2005 |
| D & L Property Solutions Ltd | Consett | 2019 |
| D a Davies Offshore Limited | Lowestoft | 2024 |
| D a Sales Limited | Halifax | 2002 |
| D C Material Management Limited | Hartlepool | 2019 |
| D Callaghan WS Ltd | Peterhead | 2023 |
| D Davies Limited | Lowestoft | 2022 |
| D F Rope Access & Rigging Ltd | Great Yarmouth | 2025 |
| D Gibson Contracting Ltd | Uttoxeter | 2023 |
| D Holley Process Services Ltd | Stockton-On-Tees | 2018 |
| D Jackson Construction Ltd | Lowestoft | 2025 |
| D Light Access Ltd | Wirral | 2011 |
| D P Squire Limited | Lowestoft | 2025 |
| D P Williams Group Limited | Mold | 2014 |
| D P Williams Quarries Limited | Mold | 2019 |
| D Potts Electrical Engineering Ltd | Sunderland | 2010 |
| D T Services 30 Limited | Beccles | 2016 |
| D W Irving Ltd | Diss | 2013 |
| D&A Sand & Gravel Ltd | Cookstown | 2024 |
| D&C Flooring Specialist Limited | Aberdeenshire | 2021 |
| D-Side Hvac Ltd | Stonehaven | 2020 |
| D. Acheson (Navan Quarries) Limited | Cookstown | 1956 |
| D. Acheson (Pressed Brick) Limited | Cookstown | 1960 |
| D. Geddes (Contractors) Limited | By Arbroath | 1976 |
| D. J. Wood Limited | Saltburn-By-The-Sea | 2016 |
| D.B. Standing & SON Ltd | Milton Keynes | 2002 |
| D.Milne Services Ltd | Glasgow | 2018 |
| D13 O&G Ltd | Dundee | 2019 |
| DAB Access & Rigging Ltd | Hurlford | 2024 |
| DAB Geotechnics Ltd | Morpeth | 2013 |
| DAB Well Operations Ltd | Montrose | 2011 |
| Dalcros (Fraserburgh) Ltd | Fraserburgh | 2024 |