| Arden Photonics Ltd | West Midlands |
| Arden Software Ltd | Greater Manchester |
| Arden University Ltd | |
| Ardentisys Ltd | Bedfordshire |
| Ardes | South Yorkshire |
| Ardlin Systems Ltd | Berkshire |
| Ardnet Online Ltd | Belfast |
| Arecor | |
| Arena DM | |
| Arena Group Ltd | West Yorkshire |
| Arete Research Services, LLP | |
| Argans | |
| Argent BioPharma Ltd | |
| Argent Labs Ltd | |
| Argentics | |
| Argo Blockchain | Greater London |
| Argo DevOps Solutions Ltd | |
| Argonaut Systems Ltd | |
| ArgoNet | Essex |
| Argosy Components Ltd | Buckinghamshire |
| Argyle Consulting Ltd | |
| Argyll Technologies | |
| Ariadne Designs | Hertfordshire |
| Ariane Medical Systems Ltd | |
| Aridhia Digital Research Environment | |
| Arise Digital Media Ltd | |
| Arishi | |
| Arixess | |
| Arjuna Technologies Ltd | Tyne and Wear |
| ARK CLS | Bedfordshire |
| Ark Computer Solutions | |
| Ark Data | |
| Ark Data Centres Ltd | |
| Ark Studios | |
| Arken Digital | |
| Arkitec Software Ltd | |
| Arkivum Ltd | |
| Arkom Creative Technology Ltd | South Yorkshire |
| Arktik | |
| Arlo | |
| Arloid Automation Ltd | |
| Armada Training Solutions Ltd | Worcestershire |
| Armadale Capital plc | |
| Armadillo New Media Ltd | Buckinghamshire |
| Armalytix | |
| Armari Ltd | Hertfordshire |
| Armerdesign | West Yorkshire |
| Armour Home Electronics Ltd | Hertfordshire |
| ArmstrongB2B | |
| Arnie Web Design | Oxfordshire |