| Cynetix Ltd | Derbyshire |
| Cynical Developer | |
| Cynosure Designs | |
| Cynozure | |
| Cypher Coders | |
| Cypher Digital | |
| Cypher Meida | |
| Cypheus Ltd | Berkshire |
| Cypralis Ltd | |
| Cyprotex | |
| CyQureX | |
| Cyrane Systems | |
| Cyscale Ltd | |
| Cyted Ltd | |
| Cytomos Ltd | |
| Cytomos Ltd | |
| Cytora | |
| CytoSeek Ltd | |
| CZDesignz | |
| D A Archive Solutions Ltd | Wiltshire |
| D H Technical Services | Kent |
| D&D Network Services Ltd | |
| D-RisQ | |
| D-TACQ Solutions Ltd | Glasgow |
| D-Tech IT | Lancashire |
| D-Wall IT Solutions Ltd | |
| D.A.T.A. Computer Services | Edinburgh |
| D.H. Software Systems Ltd | |
| d.labs | |
| D2 Interactive Ltd | |
| D3 Creative | |
| D3 Digital | |
| d3i Ltd | Buckinghamshire |
| D3R Ltd | West Sussex |
| D4t4 Solutions | Surrey |
| D55 | |
| D8 | Glasgow |
| d:code:it | |
| DA Languages | |
| DA Systems | |
| Daatrics Ltd | |
| Daatrics Ltd | |
| DaaZ Ltd | |
| DabApps | |
| DABS UK | Berkshire |
| Dacan Internet Ltd | Greater Manchester |
| DADE2 | |
| Daden Ltd | West Midlands |
| Daffodil I.T. Ltd | Cumbria |
| Daffodil IT Ltd | |