| Arnott Design Ltd | Cambridgeshire |
| Aroa Biosurgery Ltd | |
| Arpey Internet | Kent |
| Arqit | |
| Arqiva Ltd | Buckinghamshire |
| Arrayjet Ltd | |
| Arrival UK Ltd | Greater London |
| Arrive | |
| Arrow Business Communications | Surrey |
| Arrow Web Solutions Ltd | |
| Arrows Digital | |
| Arrowscape | Worcestershire |
| Art Department | |
| Art Division | |
| Art Group UK Ltd | Berkshire |
| Art of Cloud | |
| Artemis Marketing | |
| Arteon Technology | |
| Arterius | |
| Arterius Ltd | |
| Artesian Solutions | Berkshire |
| Artifax Software Ltd | Surrey |
| Artificial Labs | |
| Artificial Societies | |
| Artilab Company Ltd | |
| Artios Digital Services Ltd | |
| Artisan Apps | |
| Artisan Websites | |
| Artists & Engineers | |
| Artistweb Ltd | |
| Artlogic Media Ltd | Greater London |
| Artlook Software Ltd | Buckinghamshire |
| Artofdata.com Ltd | Berkshire |
| Arts Alliance Media Ltd | |
| Arts University Bournemouth | |
| Arts University Plymouth | |
| ArtsEd | |
| Artumi Systems Ltd | South Yorkshire |
| Artworking | |
| Arun Control Systems Ltd | |
| ARuVR | |
| ARVRtech | Greater London |
| Arwen AI Ltd | |
| Arwen Web Design | |
| ARXAX Ltd | |
| Aryusoft Corporation Ltd | |
| ASAP Computer Services | Berkshire |
| Asap.work | |
| Ascend Diagnostics | |
| Ascendancy Internet Marketing Ltd | |