| SilverMarbles | Staffordshire |
| SilverNet Ltd | Buckinghamshire |
| Silversands | Dorset |
| Simbec-Orion Group Ltd | |
| SimBin UK | Merseyside |
| Simblox | Aberdeen |
| Sime Diagnostics Ltd | |
| SIMetrix Technologies Ltd | Berkshire |
| Siminki | Greater Manchester |
| Simkin Solutions Ltd | Kent |
| Simms International | Kent |
| Simon Chester | Hampshire |
| Simon Dawes | |
| Simon Schnieders Ltd | Berkshire |
| Simple Carver Suite | North Yorkshire |
| Simple Click Solutions Ltd | |
| Simple Media Solutions Ltd | Glasgow |
| Simple PC | Nottinghamshire |
| Simple Web Design UK | |
| SimpleServers Ltd | Worcestershire |
| Simpleweb | |
| SimpleWebhosting.co.uk | Staffordshire |
| Simplified Ltd | Greater Manchester |
| Simplify Network | |
| Simplosoft | |
| Simply Better IT | Derbyshire |
| Simply Business | |
| Simply Clicks | Kent |
| Simply Digital | Dorset |
| Simply Food Solutions Ltd | |
| Simply Games | Cambridgeshire |
| Simply Personnel | Leicestershire |
| Simply Software | West Midlands |
| Simply Super Software | Warwickshire |
| Simply The Web | |
| Simplysis Ltd | Bristol |
| SimplyWiFi Ltd | Highland |
| Simpson associates | North Yorkshire |
| Simpson Soft UK Ltd | |
| Simtec Electronics | Lancashire |
| Simtech Computer Systems Ltd | Kent |
| Simul Digital | |
| Simul Software Ltd | |
| Simulant Systems Ltd | |
| Simulation Software Ltd | |
| Simulec Systems Ltd | |
| Simulistics Ltd | Edinburgh |
| SimVenture | |
| Simworx | |
| Sincordia Ltd | Greater London |