| CK Plumbers Ltd | Construction | 2022 |
| Clairity People Consulting Ltd | Professional & scientific | 2024 |
| Clare Roberts Executive Coaching Ltd | Professional & scientific | 2021 |
| Claremont Gardens (Jarvis) Limited | Construction | 2025 |
| Clark & Williams Property Ltd | Real estate | 2025 |
| Clarke & Whalen Architects Limited | Professional & scientific | 2004 |
| Class Events and Workshops Ltd | Arts & recreation | 2021 |
| Classic & Modern Automotive Engineering Ltd | Wholesale & retail | 2022 |
| Claudia Trades Limited | Information & communication | 2025 |
| Clayton Cole Limited | Professional & scientific | 2011 |
| Clear Concise Media Limited | Information & communication | 2016 |
| Clementine Consultancy Services Ltd | Professional & scientific | 2024 |
| Clementine Strategy Limited | Professional & scientific | 2013 |
| Click & Capture Limited | Professional & scientific | 2025 |
| Click RAY Ltd | Wholesale & retail | 2023 |
| Climate Assets Limited | Professional & scientific | 2022 |
| Clonroche Properties Limited | Real estate | 2025 |
| Close to Home Dog Hotel Ltd | Other services | 2023 |
| Closer Portal Ltd | Education | 2025 |
| Cloudmood Limited | Information & communication | 2024 |
| Cloudquery Limited | Information & communication | 2017 |
| Cloudvisual Ltd | Information & communication | 2014 |
| CMI Advisers Ltd | Real estate | 2021 |
| CMS Electracom Limited | Administrative & support | 2009 |
| CMS Luton Ltd | Wholesale & retail | 2023 |
| CN Plant Services Limited | Manufacturing | 2023 |
| Coach and Outsource Limited | Professional & scientific | 2025 |
| Coachedbycurt Ltd | Education | 2025 |
| Coanalytics Ltd | Information & communication | 2019 |
| Coastal Veterinary Specialists Limited | Professional & scientific | 2022 |
| COB Consultancy Services Limited | Information & communication | 2026 |
| Cobalt and Co Consulting Limited | Professional & scientific | 2025 |
| Cobalt Private Finance Limited | Professional & scientific | 2004 |
| Codeminders Limited | Information & communication | 2019 |
| Coffee Pilgrims Limited | Professional & scientific | 2022 |
| Cognizance Consultancy Limited | Information & communication | 2013 |
| Coldharbour Lane Management Company Limited | Households | 1981 |
| Coleridge Court Residents' Association Limited | Households | 1969 |
| Collect TCG Ltd | Wholesale & retail | 2026 |
| College Road ST. Albans (Management Company) Limited | Households | 1988 |
| Colne View Limited | Construction | 2020 |
| Colo Commodities London Trading Limited | Professional & scientific | 2017 |
| Colosseum Letting Limited | Real estate | 2022 |
| Colourbox Technology Ltd | Information & communication | 2006 |
| Columbus Sixty One Ltd | Financial services | 2024 |
| Commercialbydesign Ltd | Professional & scientific | 2026 |
| Common Joy CIC | Education | 2018 |
| Commonspark Ltd | Wholesale & retail | 2023 |
| Communication Business Partnership Limited | Real estate | 2001 |
| Community Challenge Events Limited | Other services | 2025 |
| Compass Twenty Ltd | Wholesale & retail | 2021 |
| Compliance Sidekick Limited | Information & communication | 2024 |
| Comsatangel Limited | Professional & scientific | 2016 |
| Connex Digital Limited | Water & waste | 2024 |
| Connor Spencer Limited | Professional & scientific | 2012 |
| Conroy Performance Limited | Education | 2021 |
| Conscious Tech Ltd | Information & communication | 2023 |
| Conspire Pictures Limited | Information & communication | 2015 |
| Consultancy for Good Ltd | Professional & scientific | 2023 |
| Cook and Party Ltd | Wholesale & retail | 2023 |
| Cooper Flexopress Ltd | Manufacturing | 2023 |
| Copperbee Management Company Limited(The) | Households | 1986 |
| Corbar Consulting Ltd | Professional & scientific | 2008 |
| Core Creative Events Ltd | Other services | 2018 |
| Corestar Partners Ltd | Professional & scientific | 2025 |
| Cornelia & Gerard Court Management Limited | Administrative & support | 1992 |
| Corona Prisms Ltd | Professional & scientific | 2023 |
| Correm Ltd | Professional & scientific | 2019 |
| Cosmos Global Ltd | Wholesale & retail | 2026 |
| Costcapx Ltd | Real estate | 2025 |
| Cotterden Vale Limited | Health & social care | 2026 |
| Counselling and Consultancy (Herts) Ltd | Other services | 2012 |
| Courtney Black App Ltd | Other services | 2020 |
| Courtney Black PT Ltd | Other services | 2018 |
| Covalence Research Ltd | Information & communication | 2014 |
| Craiglea Industries Ltd | Manufacturing | 2023 |
| Craiv Limited | Professional & scientific | 2020 |
| Crambeck Services Ltd | Administrative & support | 2019 |
| Crater Lake Company Limited | Professional & scientific | 1995 |
| Cravell Surveying Limited | Professional & scientific | 2014 |
| Cravells Court Harpenden Limited | Households | 1964 |
| Cravells Road (Harpenden) Management Co Limited | Administrative & support | 1999 |
| CRB Associates Ltd. | Information & communication | 1999 |
| Creare Dartmouth Limited | Construction | 2025 |
| Creare Limited | Financial services | 2025 |
| Creative Diversity Network Ltd | Arts & recreation | 2015 |
| Creative Stripes Limited | Manufacturing | 2019 |
| Creative Support Hub Limited | Extraterritorial | 2020 |
| Crescendo Contracts and Consultancy Ltd | Other services | 2020 |
| Crescendo Contracts and Consultancy Services Ltd | Other services | 2021 |
| Cricket Consultants Ltd | Professional & scientific | 2012 |
| Crisstell Limited | Manufacturing | 2018 |
| Crop Demetra Limited | Financial services | 2012 |
| Crop Performance Ltd | Information & communication | 2013 |
| Crosswing Homes Limited | Construction | 2017 |
| Crosthwaite Court Residents' Association Limited | Households | 1972 |
| Crown Mayfair Limited | Real estate | 2008 |
| Crown PAY Limited | Financial services | 2025 |
| Crown Personnel Services Limited | Health & social care | 2023 |
| Crownshop Ltd | Wholesale & retail | 2024 |