| Adeo Group | Glasgow |
| Adept Computer Services | |
| Adept Energy Solutions Ltd | |
| Adept IT Solutions (Norwich), LLP | Norfolk |
| Adept PC Repair | |
| Adept Power Solutions Ltd | Hampshire |
| Adept Scientific | Bedfordshire |
| AdEPT Telecom | Kent |
| Adesignday.co.uk | |
| Adhara | |
| ADI Communications | |
| Adido Ltd | Dorset |
| Adient plc | |
| Adigi Agency | |
| Adimo | |
| Aditech Ltd | Buckinghamshire |
| AdJuice Ltd | Surrey |
| Adlantic | |
| Adlantic Ltd | |
| Adludio | |
| ADM Computing | Kent |
| ADM Web Studios | Cumbria |
| ADMAN | |
| Administrate Ltd | |
| Admix | Greater London |
| Adodo Consulting Services Ltd | Nottinghamshire |
| Adopter Ltd | |
| Adoreboard | |
| AdoreGames Studio | |
| Adrenaline VC | Cambridgeshire |
| Adria Solutions Ltd | |
| Adrone Ltd | |
| AdrosWebHost | Northamptonshire |
| ADS Computers | Nottinghamshire |
| ADS Creative | North Yorkshire |
| ADS Group | |
| Adsmartway | |
| Adstorm | Staffordshire |
| Adstract | |
| ADT Ltd | Leicestershire |
| Adtec Software Ltd | North Yorkshire |
| Adthera | |
| AdTonos | |
| ADV Communications | Northamptonshire |
| Advanced Bacterial Sciences Ltd | |
| Advanced Broadcast Services Ltd | |
| Advanced Computer Software Group Ltd | Berkshire |
| Advanced Computer Solutions (Yorkshire) Ltd | East Riding of Yorkshire |
| Advanced Control Systems | Hampshire |
| Advanced Crystal Technology | Berkshire |