| IBM United Kingdom Limited | Feltham |
| Ice Hockey UK | Romford |
| Iceland Frozen Foods Ltd | Deeside |
| ICI | London |
| Icon Display Ltd | Sidcup |
| IDEAS IN MEDIA | Glasgow |
| Identity Creation for Business | Hull |
| IDS (Interactive Digital Sales) | London |
| IEM Ltd | London |
| Ignition | London |
| IKEA Ltd | London |
| iLUKA Ltd | Kingston upon Thames |
| iMediasport Arts & Entertainment | London |
| IMG | London |
| IMG Consulting | London |
| IMI plc | Birmingham |
| Impact Public Relations Ltd | Doncaster |
| Impression Sailing | Southampton |
| In Kind Direct | London |
| InBev UK Ltd | Luton |
| Inclusive Fitness Initiative | Sheffield |
| Incorporated Society of British Advertisers Ltd (ISBA) | London |
| The Incorporated Society of Musicians (ISM) | London |
| The Independent | London |
| The Independent on Sunday | London |
| Independent Publishers Guild (IPG) | Royston |
| Independent Theatre Council (ITC) | London |
| Indesit Company | Peterborough |
| ING Bank | London |
| InGolf | Richmond |
| Initiative | London |
| Inmarsat Global Ltd | London |
| Innocent Ltd | London |
| Inprints | Sheffield |
| Inspiration | London |
| Institute of Entertainment & Arts Professionals (IEAP) | Horsham |
| Institute for Sport, Parks & Leisure (ISPAL) | Reading |
| Institute of Fundraising | London |
| Institute of Sport & Recreation Management (ISRM) | Loughborough |
| Intel Corporation (UK) Ltd | Swindon |
| Intelligent Finance | Edinburgh |
| Intercontinental Hotels Group Plc | Windsor |
| Interflora | Sleaford |
| International Beverage Holdings Ltd | Airdrie |
| International Business Leaders Forum | London |
| International Tennis Federation (ITF) | London |
| Intersport | Solihull |
| Invensys plc | London |
| Invesco Perpetual | London |
| Investec Bank (UK) Ltd | London |
| IPA Institute of Practitioners in Advertising | London |
| IPC Media | London |
|
Ipsos MORI | Harrow |
| Irish Football Association (IFA) | Belfast |
| Irish Rugby Football Union (Ulster) | Belfast |
| Irish Women's Indoor Bowling Association | Lisburn |
| Irvine Whitlock Ltd | Bedford |
| ISAF (UK) Ltd | Southampton |
| iShares | London |
| Isle of Jura Distillery | Isle of Jura |
| ITV Granada | Manchester |
| ITV Sales | London |
| Ivor Dewdney Pasties Ltd | Plymouth |
| IWK (Initiativen Wirtschaft für Kunst) | 1040 Vienna |
