| CSD Network Services Ltd | Essex |
| CSE Industrial Electrical Distributors Ltd | Buckinghamshire |
| CSE Ltd | West Sussex |
| CSG | Bridgend |
| CSI Ltd | |
| CSI Media | Cheshire |
| CSL Website Design | |
| CSP | Hertfordshire |
| CSS Wizardry Ltd | West Yorkshire |
| CST Group Ltd | East Sussex |
| CSU Web Design | |
| CSY Retail Systems Ltd | Nottinghamshire |
| CT Computer Services | |
| CT Networks | |
| CTA Systems I.T. Solutions Ltd | |
| CTCentral | Hampshire |
| CTi Communications Ltd | Gloucestershire |
| CTi Computers Ltd | Greater Manchester |
| CTI Digital | Greater Manchester |
| CTI Holdings | |
| CTMS | Berkshire |
| CTO Academy Ltd | |
| Ctrack | West Yorkshire |
| Ctrl Alt Infrastructure Solutions | |
| CTRLS | |
| CTS PC Solutions Ltd | |
| CU Marketing Ltd | Shetland |
| Cube Clean Tech Ltd | |
| CUBE Content Governance Global Ltd | |
| Cube Creative Ltd | Greater Manchester |
| Cube Webworks Ltd | South Ayrshire |
| Cube33 | Somerset |
| CubeCart Ltd | |
| Cubestech | |
| Cubic Motion | |
| Cubiqdesign Ltd | Suffolk |
| Cubit Technology Ltd | Essex |
| Cubix Software Ltd | County Durham |
| Cuckoo Internet Ltd | Greater London |
| Cudo Ventures | |
| Cudos Ltd | |
| Cull Micro Imaging | Merseyside |
| Cultivate Creative Ltd | |
| Culture Shift | |
| CultureAI | |
| Cultured Digital Ltd | |
| CultureMee | |
| Cumbria Web Services | Cumbria |
| Cumulus Neuroscience Ltd | |
| CUPE International | Dorset |