| AdTonos | |
| ADV Communications | Northamptonshire |
| Advanced Bacterial Sciences Ltd | |
| Advanced Computer Software Group Ltd | Berkshire |
| Advanced Computer Solutions (Yorkshire) Ltd | East Riding of Yorkshire |
| Advanced Control Systems | Hampshire |
| Advanced Crystal Technology | Berkshire |
| Advanced Data Recovery | Greater London |
| Advanced Electric Machines Ltd | |
| Advanced Embedded Solutions Ltd | Dorset |
| Advanced Fibreoptic Engineering Ltd | Oxfordshire |
| Advanced Mechanical Systems Ltd | |
| Advanced Research and Technology Ltd | Nottinghamshire |
| Advanced Resource Managers Ltd | Hampshire |
| Advanced Technologies International | Berkshire |
| Advanced Web Designs Ltd | Hertfordshire |
| Advansys Ltd | Berkshire |
| Advanta Productions Ltd | Nottinghamshire |
| Advantage Business Components Ltd | Hertfordshire |
| Advantage Business Systems Ltd | |
| Advantage Services (Europe) Ltd | West Midlands |
| Advantage Web Designs | North Yorkshire |
| Advantex Network Solutions Ltd | Tyne and Wear |
| AdvantureSoft | West Midlands |
| Advanz Pharma | |
| Advatech Healthcare Europe | |
| Advent IM Ltd | West Midlands |
| Advent Life Sciences, LLP | |
| Adventure Game Studio | |
| Advenzia Ltd | |
| Adviser Cloud | |
| Advoco Solutions Ltd | Berkshire |
| ADX Solutions Ltd | |
| ADZ Creative & Digital Ltd | |
| Adzact | |
| Adzuna | Greater London |
| Aegiq Ltd | |
| AEI Security & Communications Ltd | East Sussex |
| AEIOU Consulting Ltd | Buckinghamshire |
| AEM Systems | West Yorkshire |
| Aeon Design Web Solutions | South Yorkshire |
| Aerial Aid Ltd | |
| Aerial Cable and Equipment Ltd | Gloucestershire |
| Aerial Digital Ltd | Glasgow |
| Aerial Power Ltd | |
| Aerialforce | |
| Aerian | Wiltshire |
| Aero Designs | Glasgow |
| Aero IT Cloud | |
| AeroCloud | |