| Aerious | |
| Aero Designs | Glasgow |
| Aero IT Cloud | |
| AeroCloud | |
| Aeroplane | |
| Aerospace Xelerated | |
| Aerovision IT | Fife |
| AESIN | |
| Aesthetic Web Designs | |
| Aeternum Software Ltd | |
| Aetherweb Ltd | Devon |
| Aethox Therapeutics | |
| AF ChemPharm | |
| AF Oliver | |
| AFD Software | Isle of Man |
| Afentis Forensics | Merseyside |
| Affiliate Marketing | Norfolk |
| Affinitive IT Ltd | Berkshire |
| Affinity IT | South Yorkshire |
| Affinity Ventures | |
| Affordable-Websites.co.uk | |
| Affordablemobiles.co.uk | |
| aFinite | West Yorkshire |
| Afiniti Global Ltd | |
| AFIVE Digital Agency | |
| AFM Ltd | |
| Afon Technology Ltd | |
| Aframe | Greater London |
| Africa Mobile Networks Ltd | |
| Africa New Energies | |
| African Private Equity and Venture Capital Association | |
| Afterthought Software Ltd | |
| AG Grid Ltd | |
| Agamik Ltd | West Lothian |
| Agathos Systems | Nottinghamshire |
| Age Partnership Ltd | |
| AgeCurve | |
| aGeek | |
| Agena Ltd | Cambridgeshire |
| Agency Leeds | |
| Agency97 | Greater London |
| Agent8 Design Ltd | |
| AgentOS | Cardiff |
| AgentPro | |
| Aggreko | |
| Agile Analog Ltd | |
| Agile App Co. | |
| Agile Applications Ltd | |
| Agile Automations | |
| Agile Collective Ltd | |