| Crowson Consultancy Ltd | Professional & scientific | 2022 |
| CRT Plumbing & Heating Ltd | Other services | 2016 |
| CS Taxation Limited | Professional & scientific | 2015 |
| Csbd Holdings Ltd | Construction | 2022 |
| CSD Harpenden Ltd | Hospitality | 2024 |
| CST Skincare Ltd | Health & social care | 2012 |
| CT Osteopathy Limited | Health & social care | 2025 |
| Ctrlstudio Ltd | Professional & scientific | 2025 |
| Cubed Capital Management Limited | Professional & scientific | 2020 |
| Cubed Consultancy Limited | Professional & scientific | 2020 |
| Cubed Facility Management & Maintenance Limited | Construction | 2023 |
| Cubed Group Holdings Limited | Financial services | 2022 |
| Cubed Group Vision Limited | Health & social care | 2021 |
| Cubed PAY Limited | Financial services | 2025 |
| Cubed Payroll Solutions Limited | Wholesale & retail | 2023 |
| Cubed Racing Limited | Extraterritorial | 2020 |
| Cubed Studios Limited | Information & communication | 2023 |
| Cubedx Limited | Professional & scientific | 2016 |
| Cujo Ltd | Hospitality | 2015 |
| Cumberland Consultants Limited | Professional & scientific | 2011 |
| Cumberland Court (ST Albans) Management Limited | Households | 2026 |
| Curley & Maio Limited | Financial services | 2024 |
| Curtis Analytics Limited | Manufacturing | 2017 |
| Custard and Crumble Ltd | Information & communication | 2011 |
| Custom Cast Products (R.O.W) LLP | | 2009 |
| Cutaneous Limited | Health & social care | 2015 |
| CWC Construction Limited | Construction | 2018 |
| Cyber Security Case Studies Ltd | Professional & scientific | 2017 |
| Cyferkon Limited | Extraterritorial | 2012 |
| Cyku Bioplastics Limited | Manufacturing | 2021 |
| CYPHER22 Ltd | Administrative & support | 2022 |
| D & a Purkis Ltd | Professional & scientific | 2026 |
| D & R Management Ltd | Professional & scientific | 2013 |
| D C and J Ventures Limited | Professional & scientific | 2016 |
| D M Music Limited | Manufacturing | 2008 |
| D&A Intelligence Ltd | Information & communication | 2024 |
| D.T.A Cleaning & Co Ltd | Administrative & support | 2022 |
| D2 Design Consultancy Ltd | Professional & scientific | 2022 |
| Dabse Ltd | Real estate | 2021 |
| DAC Medical Limited | Professional & scientific | 2020 |
| Dagobah Tavern Limited | Hospitality | 2026 |
| Damft Capital Limited | Wholesale & retail | 2005 |
| Daniel Craig Hair Salons Limited | Other services | 2021 |
| Daniel Ramsay Kitchens Limited | Construction | 2018 |
| Daniell Limited | Construction | 2013 |
| Danik Group Technology Services Ltd | Construction | 2025 |
| Danik Technology Services Ltd | Administrative & support | 2024 |
| Darchini Harpenden Limited | Hospitality | 2025 |
| Dare Solutions Company Ltd | Wholesale & retail | 2020 |
| Dare Theatre Ltd | Arts & recreation | 2024 |
| Dark Circle Group Ltd | Other services | 2018 |
| Dark Horse Coaching Ltd | Professional & scientific | 2025 |
| Data ELT Limited | Professional & scientific | 2024 |
| Datascape Limited | Information & communication | 2024 |
| Dave Forde Consulting Ltd | Professional & scientific | 2014 |
| David Guise Associates Ltd | Professional & scientific | 2019 |
| David Horton Consulting Ltd | Professional & scientific | 2023 |
| David Kent Limited | Information & communication | 2011 |
| David Michael Consulting Ltd | Professional & scientific | 2024 |
| David Salmon Investments Limited | Professional & scientific | 2016 |
| Davis Fitness Limited | Arts & recreation | 2017 |
| Davis&Co Ltd | Professional & scientific | 2026 |
| DAWN7 Ltd | Wholesale & retail | 2025 |
| Day 5 Architecture Ltd | Professional & scientific | 2019 |
| Dayspring Investments Limited | Professional & scientific | 2002 |
| DB Software Limited | Information & communication | 2025 |
| DBW Digital Ltd | Professional & scientific | 2020 |
| DBW Professional Ltd | Professional & scientific | 2010 |
| DCM Vintners Ltd | Wholesale & retail | 2023 |
| Dcmy Property Co. Limited | Real estate | 2019 |
| DCS Smart Control Systems Limited | Manufacturing | 2021 |
| Dead Ernest Limited | Professional & scientific | 2012 |
| Deanspires Limited | Real estate | 1977 |
| Deborah Fitz Interiors Limited | Professional & scientific | 2016 |
| Decarboncities Ltd | Professional & scientific | 2024 |
| Decisionstring Limited | Information & communication | 1990 |
| Decode Marketing Limited | Professional & scientific | 2009 |
| Decopica Ltd | Arts & recreation | 2021 |
| Deeks Electrical Contractors Limited | Construction | 1983 |
| Deeks Renewables Limited | Construction | 2024 |
| Deerswood (2) Ltd | Real estate | 2020 |
| Delegate AI Services Ltd | Information & communication | 2025 |
| Deli-Fayre Ltd | Wholesale & retail | 1999 |
| Delicious and Real Ltd | Hospitality | 2019 |
| Dellcroft Way Harpenden Management Company Limited | Households | 2015 |
| Delphine Alexander Limited | Wholesale & retail | 2013 |
| Delta Heed Limited | Information & communication | 2014 |
| Delta Technics (UK) Limited | Professional & scientific | 2025 |
| Deltalux Cleaning Ltd | Administrative & support | 2013 |
| Deltalux Ltd | Construction | 2018 |
| Demandco Ltd | Professional & scientific | 2021 |
| Denise Language Services Ltd | Professional & scientific | 2021 |
| Deplidge International Limited | Arts & recreation | 2001 |
| Deriston Ltd | Wholesale & retail | 2021 |
| Dermagraph Limited | Other services | 2014 |
| Design Tomorrow Limited | Professional & scientific | 2022 |
| Designer Property Ltd | Real estate | 2016 |
| DESIGNER4LONDON Ltd | Professional & scientific | 2019 |
| Despina Property Projects Ltd | Professional & scientific | 2018 |
| Development Unlocked Limited | Professional & scientific | 2024 |