| Altapplabs | |
| Altar | |
| Altcom Ltd | Cornwall |
| Alternative Communication Systems | Oxfordshire |
| Alternative Solutions Ltd | Channel Islands |
| Altia Solutions Ltd | |
| Altify | |
| Altinet | |
| Altitide Digital Solutions Ltd | |
| Altitude Angel Ltd | |
| Altitude Interactive | |
| Alto Software | |
| Altodigital | West Midlands |
| Altodigital Networks Ltd | |
| Altrico Ltd | Greater London |
| Altrigen | West Yorkshire |
| Altron Communications Equipment Ltd | Carmarthenshire |
| Altruistiq | |
| Altum Digital Marketing Consultancy | Derbyshire |
| Altus IT Ltd | |
| Alucard Inchoate Ltd | Greater Manchester |
| Alvechurch Data Ltd | West Midlands |
| Alveo Technology | |
| AlveoGene | Oxfordshire |
| Alvi Pixels Creative | |
| Alvi Pixels Design Studio | West Yorkshire |
| Always PC | Greater London |
| AM Labels Ltd | Northamptonshire |
| AM Robotics 2018 Ltd | |
| am:pm graphics | Derbyshire |
| Amadeus Capital Partners Ltd | |
| AmaiZonia | |
| Amantys Ltd | Cambridgeshire |
| Amarin Corporation plc | |
| Amaris Web Design | |
| Amas Murrison Ltd | |
| Amasci Creative | Buckinghamshire |
| Amati Global Investors | |
| Amatis Networks | Berkshire |
| Amato Capital | |
| Amazing Domains | |
| Amba Consultants | Greater London |
| Ambar Cloud Ltd | |
| Ambeon Capital plc | |
| Amber Therapeutics | |
| AmberCouch | Rhondda Cynon Taf |
| Ambersail | Cheshire |
| Ambersoft | Bristol |
| Ambiental | |
| Ambos Digital Ltd | |