| Clicks Marketing | Cambridgeshire |
| ClickThrough Marketing | Staffordshire |
| Clicky Media Ltd | Cheshire |
| Client Software Ltd | |
| CLIFF Electronic Components Ltd | Surrey |
| Clifford Chance | |
| Clifton IT | Bristol |
| Clifton Media | Derbyshire |
| Clifton Web Design | Bristol |
| Climate Connect | Greater London |
| Climate X Ltd | |
| Climate17 | |
| ClimateAligned | |
| Climb Digital | |
| Climb Online Ltd | |
| Climber.eu | Greater London |
| Clinical Design Technologies Ltd | |
| Clinical Metadata | |
| Clinical Supplies Management | |
| clinked.com | |
| Clive Angel | |
| Clock Software | Greater London |
| clock.bio | |
| Clockstudio | |
| ClockTowerWeb | Berkshire |
| Clockwork Bear | Swansea |
| Clockwork Creative Technology Ltd | |
| Clockwork Marketing | Devon |
| Cloobees UK Ltd | |
| Close Brothers Group plc | |
| Closed Loop Medicine | |
| Closir | |
| Cloud & Cyber Security Expo | Greater London |
| Cloud AI Tech | |
| Cloud and More | |
| Cloud Capture Ltd | |
| Cloud Computing News | Bristol |
| Cloud Cover | |
| Cloud Cover Hosting Ltd | Cornwall |
| Cloud Create | |
| Cloud Cycle Ltd | |
| Cloud Employee | |
| Cloud Energy | |
| Cloud Essentials | Somerset |
| Cloud Fulfilment | |
| Cloud Global | |
| Cloud Industry Forum | |
| Cloud Maker | |
| Cloud Orca Ltd | |
| Cloud Precision | |