| Catalyst Websites Ltd | South Yorkshire |
| Catalyst, a Constellation Software Company | Leicestershire |
| Catalyst2 | Belfast |
| Catalyx | |
| Catapult Network | |
| Catapult Ventures | Leicestershire |
| CatBase Publishing Systems Ltd | Hertfordshire |
| Catch Designs Management | |
| CatchApp | |
| CaterBook | |
| Cathedral Web Design | Lincolnshire |
| CatSci Ltd | |
| Cattle Eye Ltd | |
| Cattleman Technologies Ltd | |
| Caura | Greater London |
| Causa Ltd | |
| causaLens | |
| Causaly | |
| Causeway Sensors Ltd | |
| Causeway Technologies Ltd | Buckinghamshire |
| Cavendish | Monmouthshire |
| Cavendish Communications | East Sussex |
| Cavitas Energy Ltd | |
| Cayenne Red | Berkshire |
| Cayenne Web Development Ltd | Oxfordshire |
| Cayman National - Isle of Man | |
| Cayman Venture | Tyne and Wear |
| Caystard Group International | |
| Caz Ltd | Bristol |
| Cazbah Ltd | Cardiff |
| CB Digital Marketing | |
| CB IT Consultancy | |
| CB Technology Ltd | |
| CBC Computer Systems Ltd | South Yorkshire |
| CBCOM | Merseyside |
| CBL Data Recovery (Newcastle) Ltd | Tyne and Wear |
| CBRE | |
| CBS Business Consulting Ltd | |
| CBS Solutions Ltd | |
| CBT Solutions Ltd | Buckinghamshire |
| CC Electronics Ltd | Cheshire |
| CC Grant Tracker | Glasgow |
| CCC IT Solutions | Hampshire |
| CCL (Solutions) Group Ltd | Buckinghamshire |
| CCl Components Ltd | Glasgow |
| CCL Computers Ltd | West Yorkshire |
| CCL Logistics | |
| CCm Technologies | |
| CCQ Tech | |
| CCR Systems (UK) Ltd | West Yorkshire |