| Cozens Road Management Limited | Households | 1970 |
| Crab Race Limited | Information & communication | 2013 |
| Crackerjack (UK) Ltd | Professional & scientific | 2012 |
| Craftmax Estates Limited | Real estate | 2001 |
| Craighton Solutions Ltd | Professional & scientific | 2024 |
| Cranley Gallery Limited | Wholesale & retail | 2001 |
| Crannis & Co Limited | Professional & scientific | 2013 |
| Crate Hire Group Ltd | Administrative & support | 2026 |
| Craven Commercial Finance Limited | Financial services | 2023 |
| Craven Financial Limited | Financial services | 2013 |
| Crawley Estates Limited | Real estate | 2024 |
| CRC Commerce Ltd | Administrative & support | 2012 |
| CRE8TIVECARE Ltd | Health & social care | 2021 |
| Createdit Limited | Information & communication | 2020 |
| Creative Happy Place Ltd | Administrative & support | 2024 |
| Creatively Awesome Ltd | Wholesale & retail | 2023 |
| Creditserve Business Information Limited | Professional & scientific | 2008 |
| Creek Retreats Ltd | Real estate | 2022 |
| Cresco Homes Ltd | Real estate | 2019 |
| Cresthaven Developments Limited | Construction | 1987 |
| Criterion Homes Limited | Administrative & support | 2011 |
| CRL Combined Services Limited | Construction | 2019 |
| CRM Physio Limited | Health & social care | 2025 |
| Croft Communications Limited | Information & communication | 2015 |
| Croft Court (Ruislip) Limited | Households | 1997 |
| Cromwell Advisory Ltd | Professional & scientific | 2025 |
| Cromwell Court Management Co. Limited | Households | 1983 |
| Crooklands House Properties Limited | Real estate | 2023 |
| Crosshall Gardens Management Limited | Households | 2008 |
| Crossoaks Properties Limited | Other services | 2002 |
| Crumps Environmental Limited | Water & waste | 2023 |
| Crumps Estates Limited | Real estate | 2023 |
| Cruxarch Ltd | Professional & scientific | 2008 |
| Crystalee Limited | Other services | 2012 |
| Crystals and Fossils Ltd | Wholesale & retail | 2018 |
| CS Rugby 1863 | Arts & recreation | 2012 |
| CSB Property Investment Ltd | Real estate | 2022 |
| CSC Controls Ltd | Construction | 2017 |
| Cshorten Ltd | Professional & scientific | 2025 |
| CTG Boxing Ltd | Arts & recreation | 2021 |
| Cubic Offsite Limited | Manufacturing | 2023 |
| Cubittnash Limited | Construction | 2021 |
| Cudos Financial Limited | Professional & scientific | 2025 |
| Cuffley Hill Preschool Ltd | Education | 2018 |
| Cuildochart Forestry Limited | Agriculture & fishing | 2019 |
| Cullen Timber Design (Oak) Limited | Professional & scientific | 2025 |
| Cullen Timber Design Limited | Professional & scientific | 2007 |
| Cult Club Coffee Ltd | Hospitality | 2025 |
| Cumberland Mews (Redbourn) Management Company Limited | Households | 2004 |
| Cummings Services Limited | Construction | 2024 |
| Currents Training Solutions Ltd | Education | 2026 |
| Curtech Services Limited | Construction | 2007 |
| Curzon Gate Court (Watford) Residents Association Limited | Households | 1990 |
| Custom Build Electrical Limited | Construction | 2024 |
| Custom Build Plumbing & Heating Limited | Construction | 2015 |
| Custom Capital Ltd | Professional & scientific | 2023 |
| Custom Credit Limited | Information & communication | 2021 |
| Custom Intelligent Security Limited | Administrative & support | 2017 |
| Custom Products Limited | Extraterritorial | 2002 |
| Custom Security Group Limited | Administrative & support | 2005 |
| Custom Security Solutions Limited | Administrative & support | 2000 |
| CUT MFG Ltd | Manufacturing | 2022 |
| Cutting Edge Surgical Instruments Ltd | Manufacturing | 2019 |
| CWB Development Consultants Limited | Construction | 2025 |
| CWF Investments Limited | Administrative & support | 2020 |
| Cyber Savvy Ltd | Information & communication | 2025 |
| CYP Limited | Information & communication | 1977 |
| CYP Music Limited | Information & communication | 1990 |
| CYP Productions Limited | Information & communication | 1990 |
| D & J Nails Limited | Other services | 2019 |
| D & L Medical Limited | Wholesale & retail | 2001 |
| D & L Vehicle Services Limited | Wholesale & retail | 2011 |
| D & S Property Holdings Ltd | Real estate | 2017 |
| D C S Events Limited | Administrative & support | 2018 |
| D Graves Marketing Ltd | Administrative & support | 2025 |
| D K L Electrical Limited | Construction | 2010 |
| D M C Plumbing & Heating Services Ltd | Construction | 2012 |
| D Michaelides Limited | Professional & scientific | 2021 |
| D P K Design Limited | Professional & scientific | 2009 |
| D Walker Mechancial Services Limited | Construction | 2021 |
| D'Belle Ltd | Real estate | 2022 |
| D-Sign Limited | Administrative & support | 2002 |
| D.E. Simpson Limited | Wholesale & retail | 2012 |
| D.G. Walker Limited | Administrative & support | 2013 |
| D.J. Commercial Cleaning Ltd | Administrative & support | 2003 |
| D.L. Graphics Limited | Manufacturing | 1991 |
| Dacdk Tech C4G Ltd | Professional & scientific | 2026 |
| Daklam Limited | Administrative & support | 2014 |
| Dalba Limited | Professional & scientific | 2023 |
| Dalbourne Limited | Households | 1978 |
| Daley Associates Ltd | Administrative & support | 2010 |
| Dallkett Construction Ltd | Construction | 2013 |
| DAM J Enterprises Ltd | Professional & scientific | 2020 |
| Dames Ltd | Transport & logistics | 2024 |
| Damp Away Property Preservation Limited | Construction | 2024 |
| Danad Art Limited | Manufacturing | 2016 |
| Danad Design Foundation | Education | 2018 |
| Danes Farm Stables Ltd | Agriculture & fishing | 2013 |
| Danielle COX Wellbeing Limited | Other services | 2025 |
| Danlill Limited | Professional & scientific | 2005 |