| Artios Digital Services Ltd | |
| Artios Pharma | Cambridgeshire |
| Artisan Websites | |
| Artists & Engineers | |
| 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 Web Design | Bristol |
| Arun Control Systems Ltd | |
| ARuVR | |
| ARVRtech | Greater London |
| Arwen Web Design | |
| Aryusoft Corporation Ltd | |
| ASAP Computer Services | Berkshire |
| Asap.work | |
| Ascend Advanced Therapies | |
| Ascension Ventures Ltd | Greater London |
| Ascensor | |
| Ascom (UK) Ltd | West Midlands |
| Ascot Business Solutions Ltd | |
| Asdon Group | Lisburn and Castlereagh |
| ASE Computer Services | North Yorkshire |
| ASG Media | South Yorkshire |
| Ash Telecom | Surrey |
| ASH Wireless Electronics Ltd | |
| Ashbrook Research & Consultancy Ltd | |
| Ashby Computer Services | Northamptonshire |
| Ashdown Solutions Ltd | |
| Ashe Consulting | |
| Ashlea Components Ltd | Wiltshire |
| Ashley Bolser Agency | West Yorkshire |
| Ashman Consulting Ltd | |
| Ashmead Designs Ltd | Devon |
| Ashton Court Group | Northamptonshire |
| Asimtek IT Services | |
| ASK Technology Ltd | Hampshire |
| ASK4 Ltd | |
| ASKE Consulting | |
| ASM Development | |
| ASM Technologies Ltd | |
| Asoko Insight | |
| Asolvi | Buckinghamshire |
| AsOne | Greater Manchester |
| Aspect IT Ltd | Greater Manchester |
| Aspect Media International | |