| ACM Solutions Ltd | |
| Acora | West Sussex |
| Acorn Web Design | Oxfordshire |
| Acquaint CRM | Oxfordshire |
| Acquire Digital | Leicestershire |
| Acquisition Systems Ltd | Berkshire |
| Acronyms Ltd | Devon |
| ACS Distance Education Ltd | West Midlands |
| ACS Office Solutions | Northamptonshire |
| ACS Solutions | |
| AcSoft | |
| ACT Systems | |
| Actable AI | |
| Actica Consulting Ltd | Surrey |
| Actimed Therapeutics Ltd | |
| Action Consulting | |
| Action Starter Ltd | |
| action.ai | Greater London |
| Actioncy | |
| ActionPoint Technology Services Ltd | Lisburn and Castlereagh |
| Actionstream | |
| ActionWest Ltd | Devon |
| Actis | |
| ACTIV Computer Services | North Yorkshire |
| Activ Web Design | West Yorkshire |
| ActivAeon Ltd | |
| Activate Digital Ltd | |
| Activate Studio | |
| Active Alliance Ltd | Cambridgeshire |
| Active Digital Communication Ltd | Kent |
| Active Electronics plc | Buckinghamshire |
| Active Housing | |
| Active Internet Marketing (UK) | |
| Active Merchant Services Ltd | |
| Active Mind Solutions | |
| Active Navigation | Hampshire |
| Active Office | East Ayrshire |
| Active Robots Ltd | Somerset |
| Active Solutions Ltd | Lancashire |
| Active Webdezign Ltd | |
| Active-PCB Solutions Ltd | Berkshire |
| ActiveOps Ltd | Berkshire |
| Activereach Ltd | |
| Activesol | Hampshire |
| Actopia | Buckinghamshire |
| Acttron Ltd | Hertfordshire |
| Acu IT Solutions | Glasgow |
| Acua Ocean | |
| AcuIT Ltd | Kent |
| Acuiti Labs | |