| Citel | Leicestershire |
| Citibank UK Ltd | Greater London |
| CitizenMe | |
| CitizenMe Ltd | |
| Citrixit Consultancy | |
| City College Norwich | |
| City Computing Ltd | Greater London |
| City Ink Express Ltd | Derbyshire |
| City Numbers Ltd | |
| City of Glasgow College | |
| City PC Repairs | Edinburgh |
| City, University of London | Greater London |
| City, University of London - Centre for Software Reliability | Greater London |
| City, University of London - London Space Innovation Centre | Greater London |
| City, University of London - School of Mathematics, Computer Science & Engineering | Greater London |
| CityFleet Networks Ltd | |
| Cityscape Digital | |
| CIVIC | Edinburgh |
| Civica | Greater London |
| Cizzle Biotech | |
| CJ Systems | |
| CJB Interactive | West Midlands |
| CJE Micro's | West Sussex |
| CJS Engineering Ltd | Gloucestershire |
| CJT Group Ltd | Essex |
| CK Systems | |
| CKS Global Solutions Ltd | Hertfordshire |
| ClamXav | Edinburgh |
| Claranet Ltd | Greater London |
| Claranet Ltd | Greater London |
| Claranet SOHO | Cheshire |
| Clare Associates Ltd | Cornwall |
| Clarilis Ltd | |
| Clarion Communication Systems Ltd | |
| Claritech Ltd | |
| Clarity Ltd | |
| Clarity Software | Staffordshire |
| Clarivate | |
| Clark Communications Ltd | |
| Clark Electronic Consultants Ltd | Stirling |
| Clark Studios Ltd | |
| Clarke Computer Systems | |
| Clarke Telecom | |
| Clarkson Computer Sales Repairs | |
| Claro Software Ltd | Lancashire |
| Claromentis | East Sussex |
| Clarydon Electronic Services Ltd | West Midlands |
| Class Systems | Dorset |
| Class Technology Solutions | |
| ClassForKids | |