| Annexio Ltd | Isle of Man |
| Annodata Ltd | Hertfordshire |
| Anopia Digital | |
| Anorak | Greater London |
| Anorak Technologies Ltd | Greater London |
| Another Round | |
| ANS Group | Greater Manchester |
| Answer Digital Ltd | West Yorkshire |
| Answers 2000 Ltd | Buckinghamshire |
| Answers 2000 Ltd | |
| ANT Telecom | Buckinghamshire |
| Antal International | |
| Antavo | |
| AntennaWare Ltd | |
| Antenova Ltd | Hertfordshire |
| Antev Ltd | |
| Antex (Electronics) Ltd | Devon |
| Anthemion Software Ltd | Edinburgh |
| Antibodies.com | |
| Antibody Analytics Ltd | |
| Antibody Production Services | |
| Antikor | |
| Antiquity Software | Hertfordshire |
| Antiverse | |
| Antivirus Market Ltd | |
| Antser Group | |
| Antura | |
| Anuitex Solutions Ltd | |
| Anvil | |
| Anvil MCS Ltd | Hampshire |
| Anymail Finder | |
| Anything Ltd | |
| AO Retail Ltd | |
| Aon plc | |
| AP Benson | Greater London |
| Apache | Hertfordshire |
| Apadmi Ltd | Greater Manchester |
| Apasphere Ltd | Cambridgeshire |
| APC Communications Solutions Ltd | Essex |
| APC Technology Group plc | Kent |
| APD Communications Ltd | East Riding of Yorkshire |
| Ape Red Media | Hampshire |
| Aperta | |
| Apertis Consulting Ltd | Glasgow |
| APEX Computing Ltd | Greater Manchester |
| Apex Electronics | East Sussex |
| Apex Lab Ltd | |
| Apex Molecular Ltd | |
| Apex Networks Ltd | Kent |
| Apex Robot Systems | Buckinghamshire |