| Clark Dental Equipment Systems Limited | Wholesale & retail | 1975 |
| Class Panelcraft Limited | Wholesale & retail | 2004 |
| Class Plumbing & Heating Limited | Construction | 2004 |
| Classics 'N' Collectables Limited | Wholesale & retail | 2005 |
| Claudelci Ltd | Manufacturing | 2025 |
| Claudius Designs Limited | Construction | 2007 |
| Clauson Flat Management Company Limited | Real estate | 1982 |
| Clayhall Limited | Hospitality | 2015 |
| Clean Energy Maintenance Ltd | Manufacturing | 2023 |
| Cleaner Air Limited | Other services | 2016 |
| Cleanflow Ltd | Information & communication | 2025 |
| Clear Blue Systems Limited | Information & communication | 1997 |
| Clear Financial Holdings Limited | Administrative & support | 2006 |
| Clear Heads Digital Consultants Ltd | Professional & scientific | 2018 |
| Clear Pipe Limited | Water & waste | 2007 |
| Cleardean Limited | Administrative & support | 1979 |
| Clearview Communications Limited | Manufacturing | 1996 |
| Clearview Driving Ltd | Education | 2023 |
| Clearway Finance Company Limited | Financial services | 1968 |
| Cleaver Finishings Limited | Construction | 2004 |
| Cleeve House Care Limited | Health & social care | 2007 |
| Clematis Management Company Limited | Households | 1985 |
| Clever Foods International Ltd | Wholesale & retail | 2025 |
| Click Hearing Limited | Health & social care | 2002 |
| Click NEX Ltd | Wholesale & retail | 2026 |
| Clicknbuytoy Ltd | Wholesale & retail | 2025 |
| Clientact Public Relations Ltd | Professional & scientific | 1994 |
| Cliff Hills Limited | Construction | 2015 |
| Cliff London Limited | Construction | 2004 |
| Clifton Collective Limited | Real estate | 2001 |
| Climpson Developments Limited | Construction | 2007 |
| Clinic Flow Limited | Health & social care | 2025 |
| Clinical Projects International Limited | Professional & scientific | 2026 |
| Clinical Resolutions Ltd | Professional & scientific | 2007 |
| Clinico Cleaning Services Ltd | Administrative & support | 2017 |
| Clinteach Studio Ltd | Information & communication | 2026 |
| Clintons Management Limited | Construction | 1993 |
| Clive Atkins Consultancy Limited | Education | 2003 |
| Clive Beardall Restorations Limited | Other services | 2003 |
| Clobak Consultancy Ltd | Information & communication | 2006 |
| Closeaim Limited | Construction | 2006 |
| Closecourtt Limited | Financial services | 1986 |
| Cloud Cove Limited | Information & communication | 2024 |
| Cloud Force Technologies Ltd | Information & communication | 2026 |
| Cloudcore Digital Solutions Ltd | Information & communication | 2023 |
| Cloudsell Ltd | Wholesale & retail | 2026 |
| Cloudspart Limited | Wholesale & retail | 2026 |
| Cloudspire Ltd | Wholesale & retail | 2025 |
| Clovelly Homes Limited | Construction | 2007 |
| Cloverleaf Design Services Ltd | Wholesale & retail | 1996 |
| CLS Electrical Limited | Construction | 1998 |
| Club Catering | | 1993 |
| Club Woodham Ltd | Arts & recreation | 2006 |
| Clubtokyo Ltd | Wholesale & retail | 2019 |
| Cluny Croft Residents Limited | Households | 2002 |
| Clyo Ltd | Other services | 2025 |
| Cmes Properties Ltd | Real estate | 2024 |
| Cmmtsb Limited | Wholesale & retail | 1991 |
| CMR Transport Limited | Transport & logistics | 2025 |
| CMS Corporate UK Ltd | Real estate | 2023 |
| Cmtm Ltd | Information & communication | 2013 |
| Cncs Services Limited | Professional & scientific | 2005 |
| CNH Trustee Limited | Financial services | 1994 |
| CNH U.K. Limited | Manufacturing | 1986 |
| CNL Services Ltd | Construction | 2017 |
| Cnnsoiel Ltd | Wholesale & retail | 2025 |
| CNR Food Ltd | Hospitality | 2023 |
| Co-Ordinated Packaging Limited | Manufacturing | 1978 |
| Coach Zeynep GOK Ltd | Administrative & support | 2023 |
| Coates Bro's (Upminster) Ltd. | Construction | 1994 |
| Cobblestone Concrete Contracts Limited | Wholesale & retail | 1988 |
| Cobe Bar Ltd | Hospitality | 2019 |
| Coconvali Ltd | Wholesale & retail | 2024 |
| Codalb International Ltd | Wholesale & retail | 2017 |
| Codao Ltd | Information & communication | 2025 |
| Code Flick Ltd | Information & communication | 2025 |
| Coffee & Nuts Limited | Wholesale & retail | 2020 |
| Coffey Engineering Limited | Professional & scientific | 2007 |
| Cogniera Labs Limited | Wholesale & retail | 2025 |
| Cognitia Clinical Advisory Limited | Health & social care | 2026 |
| Cohen T.V. Aerials Co. Limited | Information & communication | 1972 |
| Colchester Electrical (Wholesale) Limited | Wholesale & retail | 1984 |
| Colchester Engagement & Next Steps Ltd | Hospitality | 1990 |
| Colchester Fuel Injection Limited | Manufacturing | 1976 |
| Colchester Press Limited | Manufacturing | 2002 |
| Colchester Property Management Limited | Real estate | 2003 |
| Colchester Scaffolding Limited | Construction | 2000 |
| Colchester Society of Model and Experimental Engineers Limited | Arts & recreation | 1995 |
| Colchester Tillage Limited | Extraterritorial | 1949 |
| Colchester Tractors Limited | Real estate | 1947 |
| Colchester Travel Ltd | Administrative & support | 1990 |
| Coleclass Limited | | 1979 |
| Coleman Engineering (Structural) Limited | Construction | 1990 |
| Colin Banks Chandeliers Limited | | 1985 |
| Colin Harrison & Company Limited | Professional & scientific | 2003 |
| Colin's Glass and Glazing Limited | Construction | 2006 |
| Collier & Catchpole Limited | Wholesale & retail | 1975 |
| Collins Grose Limited | Professional & scientific | 1969 |
| Collision Division Limited | Wholesale & retail | 2025 |
| Colne Valley Railway Enterprises Limited | Transport & logistics | 1999 |