| IDA Foundation | Professional & scientific | 2015 |
| Idea MAN Films Ltd | Information & communication | 2019 |
| Ideas Industry (Reading) Limited | Administrative & support | 2007 |
| IDS Plumbing and Heating Services Ltd | Construction | 1998 |
| IF Security Ltd | Information & communication | 2023 |
| Ifac Services UK Ltd | Professional & scientific | 2024 |
| IJ Agricultural Fencing Services Ltd | Agriculture & fishing | 2011 |
| Ijmdesign (UK) Ltd | Professional & scientific | 2006 |
| Ilchester Court Management Limited | Households | 1984 |
| Iliffe News and Media Limited | Financial services | 1961 |
| Ilkram Limited | Professional & scientific | 2007 |
| Illist Ltd | Wholesale & retail | 2021 |
| Illsley (Chieveley) Limited | Agriculture & fishing | 1957 |
| Illuminate Psychotherapies Ltd | Health & social care | 2026 |
| Ilsley Engineering Limited | Professional & scientific | 2012 |
| Ilsley Holdings Limited | Administrative & support | 1983 |
| Imago 2014 Limited | Wholesale & retail | 1991 |
| Imhotek UK Limited | Information & communication | 1985 |
| Imint Ltd | Wholesale & retail | 2025 |
| Immunobiology Limited | Professional & scientific | 1999 |
| Imorphics Limited | Information & communication | 2001 |
| Impact Contract & Commercial Ltd | Professional & scientific | 2023 |
| Impactgraphics Limited | Professional & scientific | 1999 |
| Impacz Properties Ltd | Real estate | 2026 |
| Imperial Contract Cleaning Limited | Administrative & support | 2012 |
| Imperial Court (Newbury) Management Limited | Households | 2000 |
| Imperial Oak Build & Reform Limited | Construction | 2025 |
| Imperial Place Maidenhead Freehold Company Limited | Households | 2023 |
| Impetus Capital Limited | Professional & scientific | 2006 |
| IN INK (London) Ltd | Information & communication | 2014 |
| IN Silichrom Ltd | Professional & scientific | 2020 |
| In-Line Valve Company Limited | Wholesale & retail | 2001 |
| Inaplex Limited | Administrative & support | 1999 |
| Inatech Europe Limited | Information & communication | 2002 |
| Inch Punch Ltd | Information & communication | 2019 |
| Incisive Energy Limited | Manufacturing | 2015 |
| Independent Pharma Associates Limited | Professional & scientific | 2019 |
| Indigenous Brewery Ltd | Manufacturing | 2013 |
| Indigo Bay Newbury Limited | Hospitality | 2025 |
| Indigo Bay Restaurant Ltd | Hospitality | 2026 |
| Indlovu Limited | Professional & scientific | 2018 |
| Induna Meats Limited | Manufacturing | 2017 |
| Infernum Entertainment Ltd | Information & communication | 2021 |
| Inflo Studio (Oxfordshire) Limited | Education | 2025 |
| Infodata Systems Limited | Information & communication | 1980 |
| Infogent Tech Limited | Information & communication | 2026 |
| Ingham's Nurseries Limited | Agriculture & fishing | 2022 |
| Initium Corporate Finance Limited | Administrative & support | 1994 |
| Initium Employee Ownership Trustee Limited | Administrative & support | 2022 |
| Inizio Marketing Services Limited | Professional & scientific | 2007 |
| Inkpen Construction Ltd | Construction | 2023 |
| Innocent Engineering Limited | Professional & scientific | 2015 |
| Innocent Solutions Limited | Information & communication | 2009 |
| Innopharma International UK Limited | Administrative & support | 2012 |
| Innovant Research Limited | Professional & scientific | 2000 |
| Innovaphone Limited | Professional & scientific | 2000 |
| Innovative Commerce Ltd | Information & communication | 2019 |
| Inovated Integration Ltd | Administrative & support | 2018 |
| Inovett Ltd | Information & communication | 2017 |
| Inputkey Limited | Administrative & support | 1995 |
| Inseego Wireless UK Limited | Information & communication | 2005 |
| Inspire Aviation Charter Limited | Transport & logistics | 2015 |
| Inspired Collection Limited | Wholesale & retail | 2022 |
| Inspired Dining Ltd | Extraterritorial | 2017 |
| Inspired ICT Solutions Ltd | Information & communication | 2016 |
| Instructorbase Limited | Administrative & support | 2016 |
| Insurafrica Ltd | Financial services | 2020 |
| Intamesh Limited | Professional & scientific | 2010 |
| Integrated Building Consultancy Limited | Professional & scientific | 1993 |
| Integrated Ground Solutions Limited | Administrative & support | 2016 |
| Integrated Open Network Systems Limited | Information & communication | 2002 |
| Intellectus Insurance Brokers Ltd | Financial services | 2014 |
| Intellicore Consultancy Ltd | Information & communication | 2026 |
| Intelligent Horticultural Solutions Ltd | Professional & scientific | 2017 |
| Intelligent Prototype Solutions Ltd | Professional & scientific | 2017 |
| Intelligent Retail Limited | Information & communication | 2004 |
| Intense Films Ltd | Information & communication | 2020 |
| Interakt Community Limited | Education | 2011 |
| Interim BD Ltd | Professional & scientific | 2012 |
| Intermail Limited | Administrative & support | 1996 |
| International Health Limited | Health & social care | 2016 |
| International Society of Typographic Designers Limited | Professional & scientific | 2004 |
| International Taste Solutions Limited | Manufacturing | 2009 |
| International Yacht Company (IYC) Ltd | Administrative & support | 2018 |
| Introbeats Ltd | Administrative & support | 2025 |
| Intuita Residential Ltd | Real estate | 2021 |
| Intuitive Studio Ltd | Information & communication | 2019 |
| Inventive Mind Films Ltd | Information & communication | 2019 |
| Inventoryclerk Oxford Limited | Professional & scientific | 2012 |
| Invest Property Specialists Ltd | Construction | 2010 |
| Invo Interiors Ltd | Construction | 2022 |
| Ioss Limited | Wholesale & retail | 2012 |
| IOT SIM Shop Ltd | Wholesale & retail | 2025 |
| Iris Engineering Consulting Limited | Professional & scientific | 2015 |
| Ironbark Wealth Management Limited | Professional & scientific | 2023 |
| Irongate VBL Limited | Financial services | 2025 |
| ISM360 Limited | Information & communication | 2023 |
| Ison Developments Ltd | Real estate | 2013 |
| ISS Group Ltd | Administrative & support | 2015 |
| Issos Technologies Limited | Administrative & support | 2019 |