| Classic Home Improvements (East Anglia) Limited | Construction | 2020 |
| Classic Motor Events Limited | Administrative & support | 2019 |
| Classic POT Emporium Limited | Wholesale & retail | 2003 |
| Classroom Call Education Services Limited | Administrative & support | 1996 |
| Clau Fixing Limited | Construction | 2019 |
| Claudius Court Collective Limited | Hospitality | 2014 |
| Claudius Court Management Limited | Households | 1988 |
| Clavian Ltd | Education | 2021 |
| Claws and Tails Ltd | Hospitality | 2022 |
| Clay Enterprises Limited | Real estate | 2012 |
| Claycore Contracts Ltd | Construction | 2025 |
| Claydons Colchester Ltd | Wholesale & retail | 2022 |
| Claypits Developments Ltd | Construction | 2018 |
| Clayton Wright Lawncare Ltd | Administrative & support | 2013 |
| CLD Electrical Ltd | Manufacturing | 2025 |
| Clean Earth Construction Ltd | Arts & recreation | 2022 |
| Clean Earth Limited | Arts & recreation | 1998 |
| Clean Slate Cleaning Services Limited | Administrative & support | 2015 |
| Clean-Fit Limited | Administrative & support | 2026 |
| Cleaner Cleaner Ltd | Other services | 2008 |
| Cleaners Compass Ltd | Information & communication | 2025 |
| Clear - Centre for Lived Experience & Applied Research Ltd | Health & social care | 2013 |
| Clear Coordination Ltd | Other services | 2017 |
| Clear Exit Partners Limited | Financial services | 2024 |
| Clear Haven Care Limited | Health & social care | 2014 |
| Clear Liquid Advisory Ltd | Administrative & support | 2024 |
| Clear Risk IB Limited | Financial services | 2019 |
| Clear Voice: Communication and Wellbeing Services Ltd | Health & social care | 2017 |
| Clearance Items 4 ALL Limited | Wholesale & retail | 2024 |
| Cleardrop Ltd | Information & communication | 2008 |
| Clearhaven Homes Ltd | Construction | 2017 |
| Clearhaven Limited | Real estate | 2012 |
| Clearleaf Games Ltd | Information & communication | 2024 |
| Clearmark Certified Ltd | Information & communication | 2026 |
| Clearwater Partners Limited | Professional & scientific | 2013 |
| Cleethorpes Trading Ltd | Hospitality | 2018 |
| Clement Rudnicki Limited | Wholesale & retail | 2024 |
| Clemfred Ltd | Health & social care | 2014 |
| Clempat Properties Limited | Real estate | 2017 |
| Clerisy Computing Limited | Information & communication | 2009 |
| Clever Coat Ltd | Construction | 2022 |
| CLG Property Limited | Real estate | 2023 |
| CLG Services Ltd | Professional & scientific | 2024 |
| Click Freeman IT Ltd | Information & communication | 2015 |
| Cliff Court Management Limited | Real estate | 1996 |
| Cliff View Residents Association Limited | Households | 1970 |
| Clifford Brown & Grimsey Ltd | Financial services | 2002 |
| Clifford C Palmer & Co Ltd | Professional & scientific | 2017 |
| Clifford JJ 2025 Ltd | Administrative & support | 2025 |
| Clifftown Shore Management Limited | Households | 2020 |
| Clifton Administration Limited | Administrative & support | 2011 |
| Clifton Roofing Specialists Limited | Construction | 2025 |
| Climate Acoustic Solutions Ltd | Professional & scientific | 2018 |
| Climate FAN Services Limited | Manufacturing | 2014 |
| Climate Fortress Intelligence Ltd | Professional & scientific | 2026 |
| Climate Matters Limited | Professional & scientific | 2006 |
| Clinical Infection Consulting Limited | Health & social care | 2023 |
| Clipper Retail Ltd | Wholesale & retail | 2011 |
| Clive's Autos Limited | Wholesale & retail | 2021 |
| CLK Metal Shaping and Sharpening Services Ltd | Manufacturing | 2021 |
| Clockwork Electrical Solutions Limited | Construction | 2018 |
| Cloisters Court (Rainham) RTM Company Limited | Households | 2020 |
| Cloud 9 Care Limited | Health & social care | 1995 |
| Cloud and Cable Ltd | Information & communication | 2019 |
| Cloud Career Journeys Ltd | Information & communication | 2023 |
| Cloud Consultancy Services Limited | Information & communication | 2013 |
| Cloud Facilities Ltd | Administrative & support | 2013 |
| Cloud Facilities Management Ltd | Administrative & support | 2013 |
| Cloud Facilities Solutions Ltd | Administrative & support | 2013 |
| Clouderty Limited | Professional & scientific | 2009 |
| Cloudesley Seddon Limited | Financial services | 2021 |
| Cloudfm Academy Ltd | Administrative & support | 2014 |
| Cloudfm Air & Rail Limited | Extraterritorial | 2013 |
| Cloudfm Consulting Limited | Extraterritorial | 2009 |
| Cloudfm Facilities Limited | Administrative & support | 2018 |
| Cloudfm Facilities Management Limited | Administrative & support | 2018 |
| Cloudfm Facilities Solutions Limited | Administrative & support | 2018 |
| Cloudfm Group Limited | Administrative & support | 2014 |
| Cloudfm Integrated Services Limited | Administrative & support | 2010 |
| Cloudfm Shared Services Limited | Administrative & support | 2012 |
| Cloudit Bookkeeping Limited | Professional & scientific | 2015 |
| Cloudops Technologies Limited | Information & communication | 2023 |
| Cloudserve Technologies Ltd | Information & communication | 2021 |
| Cloudverge Ltd | Information & communication | 2025 |
| Clovaks Residential Ltd | Real estate | 2024 |
| Clove & Co UK Ltd | Hospitality | 2025 |
| Clover and Co. Limited | Financial services | 2012 |
| Clover Ocean Limited | Professional & scientific | 2025 |
| Clowser Electrical Ltd | Construction | 2026 |
| CLS Aesthetics Ltd | Other services | 2022 |
| Club Colours Limited | Wholesale & retail | 1985 |
| Club Djembe Ltd. | Information & communication | 2017 |
| Clubb Creative Limited | Professional & scientific | 2024 |
| Clubclass of Colchester Limited | Transport & logistics | 2003 |
| Clueless Limited | Administrative & support | 2017 |
| Clues Communications Limited | Information & communication | 1982 |
| Clunie Medical Ltd | Health & social care | 2023 |
| Clyro Limited | Real estate | 1963 |
| CM Cabling Limited | Energy | 2017 |
| CM Electrical Essex Ltd | Construction | 2022 |