| ATTAIN Group Ltd | |
| Attio Ltd | |
| Atto | |
| Attomarker Ltd | |
| Aube Creative | |
| Aubergine 262 Ltd | Bedfordshire |
| Audacia | |
| Audacity | |
| AudaCity Capital | |
| Auddy Ltd | |
| AuDeo | Hampshire |
| Audience2Media | Greater London |
| Audiense | |
| Audio Sanctuary | Greater London |
| Audio Visual Intelligence | |
| Audioboom Ltd | |
| AudioMate | Greater Manchester |
| Audiomob | |
| Audioscenic | |
| AudioStack | |
| AudioStrip | |
| Audiotel International | Northamptonshire |
| AudioTelligence | |
| Audiowings | |
| AuditWare Ltd | |
| Audium Ltd | Oxfordshire |
| Audon Electronics Ltd | Cheshire |
| Audoo Ltd | |
| Augmented Hype | |
| Augmented Text Company Ltd | |
| Augmentum Fintech Management Ltd | Greater London |
| Augmetic | |
| Augmize | |
| Augnet Ltd | |
| Aula Education Ltd | Greater London |
| Auquan | |
| Aura Computers | |
| Aura Creative Media Ltd | North Yorkshire |
| Aura Vision Labs | |
| Aurelius Investment Advisory Ltd | |
| Auriga Kore Ltd | |
| Auris Tech Ltd | |
| Aurity | |
| Aurora Tech Support | |
| Aurora Technologies Ltd | Leicestershire |
| Aurum Biosciences Ltd | |
| Aurum Solutions | |
| Authentic Style Ltd | Dorset |
| Authentiq | |
| Author Studios Ltd | |