| Cjak Consultancy Ltd | Information & communication | 2006 |
| CJM Consultants Limited | Professional & scientific | 2003 |
| CJP Electrical Solutions Ltd | Construction | 2018 |
| Class One Construction (U.K.) Limited | | 1986 |
| Classic Distribution Limited | Other services | 2003 |
| Claudio Lugli Sportswear (UK) Limited | Wholesale & retail | 2001 |
| CLC Electrics Limited | Construction | 2004 |
| Clements Hall (Commercial) Limited | Construction | 2002 |
| Cling Film Limited | Information & communication | 1998 |
| Clipso Limited | Other services | 1978 |
| Clivanton Investments Limited | Real estate | 1959 |
| Clivnars Limited | Manufacturing | 1967 |
| Cloisters Limited | Professional & scientific | 2002 |
| Club 55 Limited | Information & communication | 2013 |
| Cogon Ltd Partnership | | 1992 |
| Cohort Security Solutions Limited | Administrative & support | 2005 |
| Coles Electroacoustics Limited | Manufacturing | 1974 |
| Collins of Harrow Limited | | 1986 |
| Commerce Computing Limited | Information & communication | 2006 |
| Communications Consultants (International) Limited | Professional & scientific | 1985 |
| Connect Electrical Contractors (Hertfordshire) Limited | Construction | 2007 |
| Connect Network Solutions Limited | Construction | 2007 |
| Consortium Freight (UK) Limited | | 1982 |
| Consult HR Limited | Professional & scientific | 2000 |
| Conty Investments Limited | Real estate | 2018 |
| Coolroe Properties Limited | Real estate | 2004 |
| Cornells Limited | Wholesale & retail | 2007 |
| Cosmographics Limited | Administrative & support | 2007 |
| Cosmos(Engineering)Limited | | 1965 |
| Costelmya Limited | Transport & logistics | 2017 |
| Countrywide Golf Contractors Ltd | Construction | 2022 |
| County & Capital Investments Limited | Real estate | 1990 |
| CP Investments (Hitchin) Limited | Financial services | 2007 |
| Creating Sales Excellence Ltd | Information & communication | 2007 |
| Cressford Associates Ltd | Professional & scientific | 2007 |
| Cross Street Centre Limited | Hospitality | 1993 |
| Crowdsurfing Ltd | Professional & scientific | 2006 |
| Croxley Hall Waters Limited | Agriculture & fishing | 1981 |
| Crusader Coaches (Harlow) Limited | | 1967 |
| CS General Partner Ltd | Administrative & support | 2025 |
| Cumberland Close Limited | Households | 2005 |
| Cumberlow Compost Services Limited | Water & waste | 2007 |
| Curium Data Systems Limited | Information & communication | 2007 |
| Custom Made Prosthetics Ltd | Health & social care | 2007 |
| Customerhouse Limited | Information & communication | 2004 |
| D&Kco Ltd | Professional & scientific | 2016 |
| D&M Freight Ltd | Transport & logistics | 2018 |
| Dacorum Council for Voluntary Service | Other services | 1983 |
| Daisy Bloom Limited | Wholesale & retail | 2001 |
| Damara Properties Ltd | Real estate | 2021 |
| DAN Floor Limited | Construction | 2005 |
| Danny Easter Property Co. Limited | | 1971 |
| Danta Construct Limited | Construction | 2025 |
| Darny Industries Limited | Professional & scientific | 2006 |
| Darrenstar Limited | | 1979 |
| Data Cabling Solutions Limited | Information & communication | 2006 |
| Datumline Construction Limited | Construction | 2005 |
| Davids Bookshops (Letchworth) Ltd | Real estate | 1963 |
| DBX Worldwide Ltd | Transport & logistics | 2019 |
| Dcfnb Ltd | Other services | 2023 |
| Dealdane Developments Limited | Real estate | 1962 |
| Dealfield Limited | Real estate | 2005 |
| Debonoir Estates Limited | Real estate | 2023 |
| Dell Projects Limited | Professional & scientific | 2002 |
| Delta X Delta P Limited | Professional & scientific | 2012 |
| Delta+ Limited | Professional & scientific | 2007 |
| DELTA83 Utilities Ltd | Construction | 2025 |
| Demetriou's Rentals Limited | Real estate | 2017 |
| Dempit Construction Limited | Construction | 1987 |
| Derek Rogers Tennis Coaching Limited | Arts & recreation | 2002 |
| Design & Model Ltd | Administrative & support | 2023 |
| Dessati Studios Ltd | Professional & scientific | 2022 |
| Dewmead Investments Ltd | Administrative & support | 2007 |
| Diamond Lettings Ltd | Real estate | 2015 |
| Direct Approach Marketing Limited | Real estate | 2005 |
| DJ Chippy Limited | Hospitality | 2006 |
| DLM Group Limited | Professional & scientific | 2018 |
| Dlqh Investments Limited | Real estate | 2018 |
| Dog Academy Limited | Other services | 2005 |
| Doherty and Lally Limited | | 1975 |
| Domain Developments Limited | | 1984 |
| Domain Finance Limited | Financial services | 2004 |
| Donald Baron Limited | Agriculture & fishing | 1939 |
| Dovechase Limited | Real estate | 1995 |
| DR a R Limited | Health & social care | 2010 |
| DR Achetu Ella Ltd | Health & social care | 2023 |
| DR Wafaa Clinic Limited | Health & social care | 2018 |
| Drain Maintenance Services Limited | Extraterritorial | 2005 |
| Dreworld Services Ltd | Information & communication | 2017 |
| Driti Respiratory Medicine Limited | Health & social care | 2024 |
| Dsigns Group Limited | Professional & scientific | 2023 |
| Duchy EYE Care Limited | Health & social care | 2007 |
| Duncan Perry Estate Agents Ltd | Real estate | 2005 |
| Dunmul Limited | | 1963 |
| Duplex Properties Limited | Real estate | 2021 |
| E and M Group Ltd | Construction | 2018 |
| E Roberts (Timber) Limited | Manufacturing | 1963 |
| E.I.C. Interior Improvements Limited | | 1978 |
| E.J. Nunn Limited | Wholesale & retail | 2002 |
| E.J.Masters Limited | Transport & logistics | 1945 |