| Alembic Design Consultants | Hertfordshire |
| Aleron Partners Ltd | |
| Alert Logic | |
| Alertacall Ltd | |
| AlertSystems Ltd | Wiltshire |
| Alesi Surgical Ltd | |
| Alessandro Isola Ltd | |
| Alex Cachia | |
| Alex Curtis | Bristol |
| Alexander & Co., LLP | |
| Alexander James Technology | Kent |
| Alexander Mann Solutions | |
| AlexSys Communications Ltd | |
| Algbra | |
| ALGLIB Ltd | |
| AlgolOnline | Ceredigion |
| Algorand Europe Accelerator | |
| Algorise | |
| Algotron Ltd | Surrey |
| Alicia Sykes | |
| Alight Solutions | |
| Aligned Assets Ltd | Surrey |
| Alinter Group Ltd | |
| Alisco IT Ltd | |
| Alissa Healthcare Research Ltd | |
| Alistair Whiteley | Somerset |
| Alistair Whiteley Web Design Ltd | |
| All Interactive Ltd | Hampshire |
| All My Ink | Norfolk |
| All My Systems Ltd | |
| All Net Solutions Ltd | |
| All Round Web Design | Derbyshire |
| All Shapes | |
| All The Top Bananas | Lincolnshire |
| All Top Notch | Cheshire |
| All Web Services Ltd | Gloucestershire |
| All.Space Networks Ltd | |
| AllCapps | |
| Allday Time Systems | Hampshire |
| Allegro Capital Ltd | |
| Allen Computer Services | |
| Allergy Therapeutics | |
| Alliance Group | North Yorkshire |
| Alliance Pharma Ltd | |
| Alliance Solutions | Hertfordshire |
| Alliance Systems Ltd | Bristol |
| Alliance Trust plc | |
| Allied Computers International Ltd | Greater London |
| Allied Telecoms Ltd | Essex |
| Allied Worldwide Ltd | Buckinghamshire |