| iPhonefixed | Cardiff |
| Iplay.com | Surrey |
| iPortalis | North Yorkshire |
| iProov | |
| Ipsirius | |
| Ipswich Computer Services Ltd | |
| Ipswich Finance Company Ltd | |
| IPTEST Ltd | Surrey |
| IPV Ltd | |
| IPXO Ltd | |
| IQ Capital Partners | |
| IQ Capital Partners, LLP | Cambridgeshire |
| IQE plc | Cardiff |
| Iquo Ltd | Hampshire |
| IQX Ltd | Scottish Borders |
| Irax Ltd | Greater Manchester |
| iRepair Studios | |
| IRIS Intelligence Ltd | Somerset |
| IRIS Software Group Ltd | Berkshire |
| Irisoft Ltd | |
| IRMA Training | East Sussex |
| IRS Marketing | Oxfordshire |
| Irvine ICT Consultancy | |
| IS People | |
| is.gd | Surrey |
| Isaac Newton Institute for Mathematical Sciences | Cambridgeshire |
| ISAI Technologies Ltd | |
| iSalon Software | |
| ISB Global Solutions | |
| ISBS Ltd | Hertfordshire |
| ISCA Web Design Ltd | Devon |
| ISEA International | |
| iService Solutions | Wiltshire |
| iSEV Ltd | Shropshire |
| iShopBuilder | West Midlands |
| Isis Innovation Ltd | Oxfordshire |
| Isis Integration Ltd | Surrey |
| ISIS Technology Solutions Ltd | West Midlands |
| iSiteTV Ltd | |
| iSIZE Ltd | |
| Island Computer Systems Ltd | Isle of Wight |
| Island Networks Group | Channel Islands |
| Island PC Services Ltd | Isle of Wight |
| Island Webservices | Isle of Wight |
| Isle Interactive | Cambridgeshire |
| Isle of Wight Computer Geek | Isle of Wight |
| Isle of Wight Web-Hosting Company | Isle of Wight |
| Ismosys Group Ltd | |
| ISMS.online | East Sussex |
| ISN Solutions Ltd | |