| Antivirus Market Ltd | |
| Antropy Ltd | |
| Antser Group | |
| Anuitex Solutions Ltd | |
| Anvil MCS Ltd | Hampshire |
| Anymail Finder | |
| Anything Ltd | |
| AO Retail Ltd | |
| Aon plc | |
| Aoplied Works | |
| AP Benson | Greater London |
| Apache | Hertfordshire |
| Apadmi Ltd | Greater Manchester |
| Apasphere Ltd | Cambridgeshire |
| Apax Partners, LLP | |
| APC Communications Solutions Ltd | Essex |
| APC Technology Group plc | Kent |
| ApconiX | |
| 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 Managed Services Ltd | |
| Apex Molecular Ltd | |
| Apex Networks Ltd | Kent |
| Apex Pro Media | |
| Apex Robot Systems | Buckinghamshire |
| Apex Techno Ltd | |
| Apexomic | |
| Apexure | |
| Apexweb | Dorset |
| APEXX Fintech Ltd | Greater London |
| APH Computers Ltd | |
| Apian Ltd | |
| APIS Assay Technologies Ltd | |
| Apis Partners, LLP | |
| APL Communications Ltd | Buckinghamshire |
| APM International | |
| Apna Developer | |
| Apnasoft Ltd | Leicestershire |
| Apoddo | |
| Apogee Corporation Ltd | Kent |
| Apogee Data Consulting | Kent |
| Apogee Solutions Ltd | Hampshire |
| Apolitical Group Ltd | |
| Apollo Engineering Consultants Ltd | |
| Apollo I.T. Ltd | Devon |