| Compucom (UK) Ltd | Construction | 2002 |
| Computask Limited | Information & communication | 2009 |
| Computer Parts Limited | Other services | 2013 |
| Conamar Building Services Limited | Construction | 1994 |
| Conamar Interiors Limited | Construction | 2005 |
| Conamar Limited | Construction | 1982 |
| Conceptonic Ltd | Information & communication | 2020 |
| Condron Investments Ltd | Real estate | 2022 |
| Conkered Limited | Wholesale & retail | 2004 |
| Connexus Event Solutions Ltd | Administrative & support | 2021 |
| Conpartium Limited | Administrative & support | 1982 |
| Constant Power Services Limited | Information & communication | 2006 |
| Constantine Bay Holdings Ltd | Financial services | 2016 |
| Constantine Plant Company Limited | Extraterritorial | 1952 |
| Contemporary Psychotherapy Ltd | Education | 2019 |
| Contract Locking Services (Holdings) Limited | Extraterritorial | 2008 |
| Control Point Solutions Limited | Professional & scientific | 2023 |
| Coolroe Management Consulting Limited | Administrative & support | 2017 |
| Copemer Ltd | Professional & scientific | 2013 |
| Core Gain UK Ltd | Professional & scientific | 2000 |
| Corneroak Management Limited | Households | 2008 |
| Corporate Training Partnerships Limited | Professional & scientific | 2002 |
| Couchman Communications Limited | Professional & scientific | 1996 |
| Coulter Kitchens Limited | Administrative & support | 2005 |
| Country Properties (Letchworth) Limited | Real estate | 2000 |
| County Leasing Systems Ltd | Professional & scientific | 2016 |
| Cozstars Technologies Limited | Wholesale & retail | 2018 |
| Cp&P Homes Ltd | Construction | 2023 |
| Cracking Recruitment Limited | Administrative & support | 2018 |
| Craig Bryder Plumbing & Heating Ltd | Construction | 2023 |
| Create Community Network CIC | Health & social care | 2016 |
| Creative INK Operations Ltd | Wholesale & retail | 2026 |
| Creative Spark Films Limited | Information & communication | 2022 |
| Crest Estimating Ltd | Professional & scientific | 2021 |
| Crestmark Developmets Ltd | Construction | 2026 |
| Cridhe Consulting Ltd | Professional & scientific | 2022 |
| Crisis Management Solutions Ltd | Professional & scientific | 2014 |
| Crossfit Beowulf Ltd | Arts & recreation | 2014 |
| Crossgate (Hitchin) Ltd | Households | 2025 |
| CSA Environmental Limited | Financial services | 2024 |
| CSA Environmental Trustee Limited | Professional & scientific | 2024 |
| CSD Automation Limited | Manufacturing | 2022 |
| Cssk EA Limited | Real estate | 2023 |
| CST Corporation Limited | Professional & scientific | 2012 |
| CST Instruments Limited | Professional & scientific | 2012 |
| CST Instruments Sales Limited | Manufacturing | 2023 |
| Curvature Group Limited | Construction | 2005 |
| Cutera Limited | Manufacturing | 2002 |
| Cutis Natural Limited | Wholesale & retail | 2021 |
| CV LAB Ltd | Information & communication | 2026 |
| CW Creative Services Limited | Professional & scientific | 2022 |
| Cygnet Investments Limited | Financial services | 2018 |
| D & M Sharp (Farming) Limited | Agriculture & fishing | 1976 |
| D & P Decorators (Letchworth) Ltd | Construction | 2021 |
| D Allen Services Ltd | Construction | 2025 |
| D Drop Consulting Ltd | Information & communication | 2018 |
| D J T Hawthorne Ltd | Financial services | 2024 |
| D M Petroleum Services Limited | Construction | 2003 |
| D&N Capitols Ltd | Hospitality | 2025 |
| D.B. Sharp and Sons Limited | Construction | 1985 |
| D.Chandler Architectural Design Limited | Professional & scientific | 2017 |
| D.Rescue Ltd | Other services | 2023 |
| DAC Industrial Steel Limited | Wholesale & retail | 2007 |
| Dacor Crew Limited | Construction | 2026 |
| Dairy Day Limited | Manufacturing | 2010 |
| Dale Hammond Associates Ltd | Administrative & support | 2022 |
| Dali Audio UK Ltd. | Wholesale & retail | 2017 |
| Dalmonds Barns Limited | Hospitality | 2001 |
| Daly & Associates Ltd | Professional & scientific | 2024 |
| Damp Proofing & Building Specialists Ltd | Construction | 2025 |
| DAN Hill Historian Limited | Professional & scientific | 2022 |
| Darnell Motors Ltd | Wholesale & retail | 2025 |
| Dartons House Management Limited | Households | 2007 |
| Dashmesh Properties Limited | Real estate | 2015 |
| Data Minds Talent Ltd | Administrative & support | 2014 |
| Datasound Laboratories Limited | Information & communication | 1991 |
| Dave Morgan Limited | Construction | 2018 |
| Davel Engineering Limited | Professional & scientific | 2007 |
| David Attwell Consulting Limited | Professional & scientific | 2025 |
| David Pittard Motorsport Limited | Arts & recreation | 2013 |
| David's Trustees Limited | Other services | 2020 |
| Davis Plastering Ltd | Construction | 2019 |
| Daytec Ltd | Information & communication | 2021 |
| Dbunz Ltd | Wholesale & retail | 2025 |
| DC Card Traders Limited | Wholesale & retail | 2025 |
| DE Havilland Wood NO. 1 Residents Company Limited | Households | 1988 |
| DE Havilland Wood NO. 2 Residents Company Limited | Households | 1988 |
| DE Manincor UK Limited | Manufacturing | 2011 |
| Dead Swanky Letchworth Limited | Other services | 2023 |
| Deal on Wheels Limited | Wholesale & retail | 2022 |
| Defensive Music Holding Ltd | Information & communication | 2024 |
| Degam Ltd | Financial services | 2010 |
| Delight Healthcare Ltd | Professional & scientific | 2009 |
| Delivery 101 Ltd | Other services | 2023 |
| Delve Interiors Ltd | Construction | 2023 |
| Dementia Consultancy Ltd | Education | 2026 |
| Denby Communications Ltd | Professional & scientific | 2018 |
| Denhigh Limited | Real estate | 1988 |
| Density Management Ltd | Professional & scientific | 2024 |
| Dentonabbey Limited | Other services | 1993 |