| Abtrace | |
| Abubble | |
| Abussi Ltd | West Midlands |
| ABVentures | |
| Abzena plc | |
| AC Computer Warehouse Ltd | Greater Manchester |
| AC Database Services Ltd | |
| AC Plus | Essex |
| Ac-Sys Ltd | West Yorkshire |
| Acacia Training Ltd | |
| Academy Networks | Berkshire |
| Acal BFi Ltd | Berkshire |
| Accappella Creative Ltd | |
| Accel-Heal Technologies | |
| Acceler8 Clinical Research Ltd | |
| Acceler8 Media Web Design | |
| Accelerate | |
| Accelerated Digital Ventures | |
| AccelerateME | |
| Accelerator Academy | |
| Accelerator London | Greater London |
| Accelerator Ltd | |
| AccelerComm Ltd | |
| AccelerComm Ltd | |
| Accelex Technology Ltd | |
| Accellabs Development | Greater London |
| Accentra Technologies Ltd | Greater London |
| Accentus Medical Ltd | |
| AcceptPhones.com | |
| Access and Security Systems Ltd | Berkshire |
| Access Computing Solutions Ltd | Buckinghamshire |
| Access Control UK | Essex |
| Access Ltd | Berkshire |
| Access Networks Solutions Ltd | Greater London |
| Access Partnership Ltd | |
| Access Volcanic | |
| Accession Therapeutics Ltd | |
| accesso Technology Group plc | Berkshire |
| AccessPay | |
| Accessplanit | |
| Accord Healthcare | |
| AccountsIQ | Greater London |
| AccountsPortal | |
| Accrofab | |
| Accsys Accountants Ltd | |
| accSYS Digital Ltd | |
| Accunea Ltd | |
| Accupart Ltd | Greater Manchester |
| AccuraData | |
| AccuraSoft Ltd | |