| Construction Management Software Ltd | Northamptonshire |
| Construction Software Services Partnership | Greater London |
| Constructive Bio | |
| Constructive IT Advice | Hampshire |
| Consult Hyperion | Surrey |
| Consult JMT | |
| Consult Recruitment | |
| Consult Red | |
| Consumable Mad | Isle of Wight |
| Consumer Cloud Services Ltd | |
| Consumer Data Research Centre | |
| Consumer Intelligence | |
| Contact IT Solutions Ltd | Hampshire |
| Contact Media | Greater Manchester |
| Contact Online Ltd | Glasgow |
| ContainCo | West Yorkshire |
| Content Formula Ltd | |
| Content+Cloud | |
| ContentRadar Ltd | |
| Contentscape | |
| Contester Technologies | |
| ConTEXT Editor | West Midlands |
| Contexxt | |
| Contic | |
| Contilio | Greater London |
| Continental Ltd | Gloucestershire |
| Contino | |
| Continually Ltd | Belfast |
| Continuity Solutions Ltd | |
| Continulus | |
| Continum Group Ltd | Greater Manchester |
| Continuum Consulting CI | |
| Continuum Industries | |
| Continuum Industries | |
| Contrac | East Riding of Yorkshire |
| ContractPodAi | Greater London |
| Contral Instrument Services Ltd | Falkirk |
| Control Alt Delete IT | Greater Manchester |
| Control IT Solutions | Glasgow |
| Control Risks Group Holdings Ltd | |
| Control Systems Services Ltd | Torfaen |
| Conundrum Industrial Ltd | |
| Converge Challenge | |
| Converge TS | Cheshire |
| Converge.io | |
| Converged Communication Solutions | |
| Convergence | |
| Conversationware Ltd | Buckinghamshire |
| Conversion Gurus | |
| Convert Digital | |