| Astwellsoft | |
| ASWeb | |
| Asylum Interactive Ltd | Aberdeenshire |
| AT Engine Controls | |
| AT Ink Cart | |
| AT Veterinary Systems | Suffolk |
| ATAC Computers Ltd | Nottinghamshire |
| Atarim | |
| ATC Semitec Ltd | Cheshire |
| ATDBio Ltd | |
| Atech | Wiltshire |
| Atech Support | Buckinghamshire |
| Atelerix Ltd | |
| aTen Therapeutics | Edinburgh |
| ATG-IT Ltd | |
| Athena Forensics | Cardiff |
| Athlon | |
| ATL Telecom Ltd | Cardiff |
| ATLANT 3D | |
| Atlantic Vision Ltd | Derbyshire |
| Atlantic Zeiser (UK) Ltd | Hampshire |
| Atlas Business Group of Companies Ltd | Cheshire |
| Atlas Cables | East Ayrshire |
| Atlas Cloud Ltd | |
| Atlas Communications Ltd | Belfast |
| Atlas Computer Systems Ltd | |
| Atlas Datacom Ltd | Greater London |
| Atlas Mapping Ltd | |
| ATM Automation Ltd | Leicestershire |
| Atoa Payments Ltd | |
| Atom bank plc | County Durham |
| Atom Create (Scotland) Ltd | South Ayrshire |
| Atom Engine Ltd | |
| Atom Learning Ltd | |
| Atom Learning Ltd | |
| Atom Stars | |
| Atomic Digital Marketing | |
| Atomic Multimedia Consultants Ltd | Nottinghamshire |
| Atomic Software | Greater Manchester |
| Atomico | Greater London |
| Atomicom Ltd | |
| Atomik Research | |
| Atomised Co-operative Ltd | |
| Attacat Internet Marketing | Edinburgh |
| Attio Ltd | |
| Aube Creative | |
| Aubergine 262 Ltd | Bedfordshire |
| Audacia | |
| Audacity | |
| AudaCity Capital | |