| A38.com | Devon |
| a451 Digital Marketing | West Sussex |
| A4Service | |
| A8OM | |
| A9 Partnership Ltd | |
| AA Logics | |
| aa mac | Greater London |
| AA SEO & Web Design | |
| AAB Computer Solutions | Greater London |
| AAB Consulting | |
| Aabyss Ltd | Merseyside |
| AAC Clyde Space | |
| AAComputerZone.com | Hertfordshire |
| AAG IT Services | Derbyshire |
| Aapex IT | |
| Aardvark Creative | Bristol |
| Aardvark Embedded Solutions Ltd | |
| Aardvarky Media | |
| Aaron Russell | Wiltshire |
| Aask Digital | |
| AB Cartridges Ltd | Lincolnshire |
| AB Multimedia Ltd | Greater London |
| AB Precision (Poole) Ltd | Dorset |
| AB Tutor | Surrey |
| AB-CD Solutions | |
| AB5 Consulting | |
| Abacus | |
| Abacus Interactive Media Ltd | |
| Abacus Medicine Pharmaceuticals Ltd | |
| Abacus Valuers Ltd | Cornwall |
| Abadalweb | Devon |
| Abalon Ltd | |
| AbBaltis | |
| Abbexa | |
| Abbey ICT | Lancashire |
| Abbey IP Ltd | Greater London |
| AbbeyX Ltd | |
| ABBInet | Oxfordshire |
| ABC Computer Solutions | |
| ABCA Systems Ltd | |
| Abcam plc | |
| Abcnet | Greater London |
| Abcom IT Solutions | West Sussex |
| Abel DIAL | Edinburgh |
| Abel Internet | Edinburgh |
| Abel VISP | Edinburgh |
| Abelon Systems Ltd | |
| Aber Instruments Ltd | |
| Aberdeen Alarm Company | |
| Aberdeen Business Computing | Aberdeen |