| ISO Design | |
| ISOCAL (UK) Ltd | Merseyside |
| iSocket Systems Ltd | |
| Isocom Components | County Durham |
| ISOCOM Ltd | |
| Isogenica Ltd | |
| Isograph Ltd | Cheshire |
| Isoline Comms | Greater London |
| IsomAb Ltd | |
| Isomer Capital | |
| Isomerase | |
| Isomerase Therapeutics | |
| Isometric HQ Ltd | |
| Isomorphic Labs Ltd | |
| iSOS.com Ltd | East Sussex |
| isoTracker Solutions Ltd | |
| ISP International Space Propulsion Ltd | |
| ISQ Crowdfunding | |
| IST Reading Ltd | Berkshire |
| ISTEK UK Ltd | Wiltshire |
| Istesso | |
| iStoreBuilder | West Midlands |
| Isulu | Derbyshire |
| ISV Software Ltd | Hampshire |
| Isys Computer Services Ltd | Falkirk |
| IT Ambulance Ltd | Wiltshire |
| IT Architecture Ltd | Greater London |
| IT Bites Ltd | Essex |
| IT Business Solutions Ltd | South Yorkshire |
| IT Clear | Shropshire |
| IT Cybx Ltd | |
| IT Digital Ltd | Greater London |
| IT Focus Telemarketing | Wiltshire |
| IT Foundations Ltd | |
| IT Governance Ltd | Cambridgeshire |
| IT Hardware & Software Ltd | |
| IT Hotdesk | Aberdeen |
| IT Hunter | |
| IT Installations Ltd | South Yorkshire |
| IT Job Board | |
| IT Move | |
| IT Partnership | Warwickshire |
| IT Pie | |
| IT PRO | Greater London |
| IT Rec Ltd | Hampshire |
| IT Reliance Computer Services | |
| IT Remedies Ltd | Buckinghamshire |
| IT Researches | Greater London |
| IT Retail Systems Ltd | West Yorkshire |
| IT Revival Ltd | Hertfordshire |