| Argent BioPharma Ltd | |
| Argent Labs Ltd | |
| Argo Blockchain | Greater London |
| Argonaut Systems Ltd | |
| ArgoNet | Essex |
| Argosy Components Ltd | Buckinghamshire |
| Argyle Consulting Ltd | |
| Argyll Technologies | |
| Ariadne Designs | Hertfordshire |
| Aridhia Digital Research Environment | |
| Arixess | |
| Arjuna Technologies Ltd | Tyne and Wear |
| ARK CLS | Bedfordshire |
| Ark Computer Solutions | |
| Ark Data | |
| Ark Data Centres Ltd | |
| Ark Studios | |
| Arken Digital | |
| Arkivum Ltd | |
| Arkom Creative Technology Ltd | South Yorkshire |
| Arlo | |
| Arloid Automation Ltd | |
| Armada Training Solutions Ltd | Worcestershire |
| Armadale Capital plc | |
| Armadillo New Media Ltd | Buckinghamshire |
| Armadillo New Media Ltd | |
| Armalytix | |
| Armari Ltd | Hertfordshire |
| Armerdesign | West Yorkshire |
| Arnie Web Design | Oxfordshire |
| Arnott Design Ltd | Cambridgeshire |
| Aroa Biosurgery Ltd | |
| Arpey Internet | Kent |
| Arqit | |
| Arqiva Ltd | Buckinghamshire |
| Arrayjet Ltd | |
| Arrayjet Ltd | |
| Arrival UK Ltd | |
| Arrive | |
| Arrow Business Communications | Surrey |
| Arrow Web Solutions Ltd | |
| Arrowscape | Worcestershire |
| Art Division | |
| Art Group UK Ltd | |
| Artemis Marketing | |
| Arterius | |
| Arterius Ltd | |
| Artesian Solutions | Berkshire |
| Artifax Software Ltd | Surrey |
| Artificial Labs | |