| AquaQ Analytics | |
| Aquatec Group Ltd | |
| Aquila Software | Kent |
| AR Digital Solutions Ltd | Devon |
| Arabesque | |
| Arachsys Internet Services Ltd | Shropshire |
| Aragon Consulting Services Ltd | Lancashire |
| Aralia Systems | |
| Araucaria Computing | Edinburgh |
| Araxis Ltd | Isle of Man |
| Araya Ventures | |
| Arbico | Greater London |
| Arbico Computers Ltd | |
| Arbolus | |
| Arbor Technology (UK) Ltd | Buckinghamshire |
| Arborea Ltd | |
| Arborsteel Ltd | |
| ARC (UK) Systems Ltd | Essex |
| ARC Computers | Lincolnshire |
| arc IT Solutions Ltd | Staffordshire |
| ARC Marine | |
| ARC Regulatory Ltd | |
| Arc V Ltd | West Midlands |
| Arc-net | |
| Arc-net | |
| Arcade Ltd | Greater London |
| ArcadeDB | |
| Arcadia Software Ltd | Greater London |
| Arcarta | |
| Arcas Web Design | Edinburgh |
| Arcatech Ltd | Lisburn and Castlereagh |
| Arch | Tyne and Wear |
| Archangel Imaging Ltd | |
| Archangel Investors Ltd | Edinburgh |
| Archangel Lightworks Ltd | |
| Archax Ltd | |
| Archer Media | |
| Archer Software | Kent |
| Architech Club | |
| Archive Management Systems Ltd | Berkshire |
| Arcis Biotechnology Ltd | |
| Arcom IT Ltd | Surrey |
| Arcosoft | |
| Arcray Webmedia | Kent |
| Arctane Digital UK | |
| Arctic Grey Ltd | |
| Arctoris Ltd | |
| Arcturus Security | Berkshire |
| Arcus Studios | |
| Arden IT | Nottinghamshire |