| CYB Catering Ltd | Hospitality | 2014 |
| Cyber Gremlin Limited | Information & communication | 2012 |
| Cyber LEX Ltd | Information & communication | 2019 |
| Cyber Sociaty LEX UK | Information & communication | 2019 |
| Cyberwatch Limited | Administrative & support | 2024 |
| Cyclescheme Limited | Wholesale & retail | 2005 |
| Cycling Strength Coach Ltd | Health & social care | 2022 |
| Cydo Logistics Limited | Health & social care | 2017 |
| Cygnet Residential Limited | Real estate | 2025 |
| Cymbyl Ltd | Professional & scientific | 2023 |
| Czun Limited | Information & communication | 2016 |
| D & a Wisharts Services Ltd | Transport & logistics | 2011 |
| D & D Services Ltd | Construction | 2025 |
| D & S Properties London Limited | Real estate | 2014 |
| D a Toxicology Services Ltd | Health & social care | 2023 |
| D Bailey Installations Limited | Manufacturing | 2010 |
| D D C Foods Limited | Wholesale & retail | 1998 |
| D D Construction Solutions Ltd | Construction | 2014 |
| D Iacob Ltd | Other services | 2016 |
| D K Brindle & SON Limited | Construction | 2021 |
| D L Perman Services Ltd | Construction | 2014 |
| D M Hurcombe Ltd | Wholesale & retail | 2025 |
| D N K Services Limited | Construction | 2008 |
| D P Clarke Family Butchers Limited | Wholesale & retail | 1996 |
| D Waldron Brickwork and Building Services Limited | Construction | 2018 |
| D&A BEAUTY4YOU Ltd | Administrative & support | 2017 |
| D&B Steel Engineering Ltd | Manufacturing | 2008 |
| D&D Medics Limited | Health & social care | 2012 |
| D&D Trans Logistic Limited | Construction | 2016 |
| D&G Home Improvements Ltd | Construction | 2010 |
| D&G Lettings One (Central) Ltd | Real estate | 2026 |
| D&H Excavations Ltd | Construction | 2026 |
| D&J B Company Limited | Professional & scientific | 2022 |
| D&L Recycling Ltd | Water & waste | 2023 |
| D&M Motorworks Ltd | Wholesale & retail | 2020 |
| D&P Medical Limited | Health & social care | 2019 |
| D&R Ventures Ltd | Wholesale & retail | 2026 |
| D. Furnell Plant Services Limited | Transport & logistics | 1967 |
| D.A. Vision Construction Limited | Construction | 2021 |
| D.R Sports Turf Consultancy Ltd | Arts & recreation | 2024 |
| D.R. Johnson (Hemel Hempstead) Limited | Wholesale & retail | 1970 |
| D.S. Drury Limited | Transport & logistics | 1988 |
| D2 Design Studio Ltd | Professional & scientific | 2018 |
| DAA Holdings Limited | Real estate | 2026 |
| Daart Consulting Services Ltd | Professional & scientific | 2025 |
| Dacez Editor Ltd | Wholesale & retail | 2021 |
| Dacha SSI Limited | Construction | 2007 |
| Daciafootballacademy Limited | Education | 2025 |
| Dacms Limited | Other services | 2013 |
| Dacom BN Ltd | Manufacturing | 2026 |
| Dacortium | Other services | 2013 |
| Dacortium Trading Limited | Other services | 2013 |
| Dacorum Builders Ltd | Construction | 2022 |
| Dacorum Contracts Limited | Construction | 2001 |
| Dacorum District Citizens Advice Bureau | Other services | 2001 |
| Dacorum Executive Cars Ltd | Transport & logistics | 2025 |
| Dacorum Healthcare Providers Limited | Health & social care | 2014 |
| Dacorum Machinery and Steam Preservation Group Limited | Arts & recreation | 2007 |
| Dacorum Plumbing Ltd | Construction | 2026 |
| Dacorum Roofing Specialists Ltd | Construction | 2026 |
| Dacorum Training and Travel Services Limited | Administrative & support | 1989 |
| Dacorum Triathlon Club Ltd | Arts & recreation | 2019 |
| Dacry Ltd | Transport & logistics | 2015 |
| DAD Paving Ltd | Construction | 2026 |
| Dadsha Investments Limited | Real estate | 2023 |
| Dadsha Service Stations Limited | Wholesale & retail | 2022 |
| Daebak Chicken Ltd | Hospitality | 2023 |
| Daemm Ltd | Construction | 2017 |
| Daily Fresh Foods Limited | Wholesale & retail | 1997 |
| Daily Motors Ltd | Wholesale & retail | 2016 |
| Daisy Hill (Morley) Management Company Limited | Households | 2014 |
| Daisy Nook Recruitment Ltd | Administrative & support | 2022 |
| Daisy Purple Ltd | Administrative & support | 2013 |
| Daleflow Limited | Manufacturing | 2007 |
| Daltonvector Ltd | Information & communication | 2025 |
| Damian Knowles Consultancy Services Limited | Information & communication | 2019 |
| Damyanov Ltd. | Other services | 2014 |
| DAN Andreica Constructions Limited | Construction | 2017 |
| DAN Burns Ltd | Professional & scientific | 2022 |
| DAN Ceramic & Stone Ltd | Construction | 2017 |
| DAN Construction & Landscapes Ltd | Construction | 2023 |
| DAN D Entertainment Ltd | Information & communication | 2023 |
| DAN Lukas Limited | Real estate | 2021 |
| DAN Transport Services Ltd | Transport & logistics | 2016 |
| Dana's Macarons Ltd | Manufacturing | 2021 |
| Danaher Services Limited | Financial services | 2014 |
| Dandoeslights Ltd | Professional & scientific | 2019 |
| Danecourt Management (Bournemouth) Limited | Households | 2018 |
| Daned Ltd | Information & communication | 2026 |
| Danelink Ltd | Wholesale & retail | 2020 |
| Daniel Jones Construction Limited | Construction | 2014 |
| Daniel's Electrical Services Ltd. | Manufacturing | 2014 |
| Daniel.D SP Ltd | Information & communication | 2025 |
| Danieldum Transport Limited | Transport & logistics | 2022 |
| Danifast Ltd | Transport & logistics | 2017 |
| Danilo Construction Ltd | Construction | 2014 |
| Danny Close Limited | Wholesale & retail | 1998 |
| Danslow Estimating Ltd | Construction | 2025 |
| Danyc Trans Ltd | Transport & logistics | 2019 |
| Darex Transport Limited | Transport & logistics | 2015 |