| Inspire Yourself Limited | Professional & scientific | 2014 |
| Insure ID Limited | Financial services | 2025 |
| Integrated Assured Solutions Ltd | Construction | 2018 |
| Integrated Neurological Services | Health & social care | 2004 |
| Intelligent Core Limited | Information & communication | 2025 |
| Intercity Movers Ltd | Transport & logistics | 2026 |
| Interisk Limited | Extraterritorial | 2017 |
| International Circuit Events Limited | Administrative & support | 2025 |
| International Dried Fruits and Nuts Limited | Wholesale & retail | 2019 |
| International Migrants Organisation for Development and Prosperity Ltd | Education | 2019 |
| International Qualifications Limited | Extraterritorial | 2016 |
| International Trade & Renting Company Limited | Administrative & support | 2001 |
| Internet Commerce Express | | 1996 |
| Internetlogic Ltd | Information & communication | 2024 |
| Interstudies London Ltd | Education | 2014 |
| Intro Meeting Ltd | Professional & scientific | 2021 |
| Intsiba Limited | Information & communication | 2018 |
| Invigorate Roofing & Property Care Ltd | Construction | 2025 |
| IOI Consulting Ltd | Education | 2013 |
| ION Bear Consulting Ltd | Professional & scientific | 2019 |
| Ionescu Gibson LLP | | 2023 |
| Iopd Ltd | Administrative & support | 2020 |
| IP Patentica Limited | Professional & scientific | 2022 |
| Ipaj Company Ltd | Hospitality | 2020 |
| Ipbros Holding Ltd | Real estate | 2020 |
| IPF Trading Limited | Health & social care | 2010 |
| Ipnature Ltd | Administrative & support | 2024 |
| Ipsemet Ltd | Information & communication | 2025 |
| Ipso Facto Productions Limited | Information & communication | 2012 |
| IQ Policy Makers Forum Centre Ltd | Administrative & support | 2019 |
| Iridium Corporate Finance Limited | Professional & scientific | 2018 |
| Iris IT Solutions Limited | Information & communication | 2012 |
| Iron Culture Fitness Ltd | Arts & recreation | 2026 |
| IRP Technique Limited | Arts & recreation | 2005 |
| Isabel Pollen Ltd | Professional & scientific | 2026 |
| Isabel Utterson Ltd | Information & communication | 2025 |
| Isbela Ltd | Administrative & support | 2015 |
| Ishaan Estates Limited | Real estate | 2023 |
| Isherwood Property Consultants Ltd | Real estate | 2021 |
| Ishvinder Financial Associates UK Limited | Financial services | 2017 |
| Island Tea Limited | Wholesale & retail | 2025 |
| Isleworth Properties Limited | Real estate | 2018 |
| IT Fits Ltd | Wholesale & retail | 2026 |
| It's ALL About the Data Limited | Professional & scientific | 2014 |
| Italian Pizza and Grill Ltd | Hospitality | 2018 |
| Itchy Feet Theatre Ltd | Arts & recreation | 2023 |
| Ithaca Venture Consulting Ltd | Professional & scientific | 2026 |
| Ivoice Technologies Limited | Information & communication | 2011 |
| Ivor White Limited | Water & waste | 1962 |
| Ivory Estates & Management Ltd | Real estate | 2021 |
| IVY HQ Limited | Administrative & support | 2023 |
| Ixscient Limited | Professional & scientific | 2003 |
| J Boiler Installations Ltd | Construction | 2020 |
| J C J Media Limited | Information & communication | 2017 |
| J Dean Carpentry & Joinery Ltd | Construction | 2026 |
| J G Sparks Limited | Construction | 2013 |
| J H & P Limited | Wholesale & retail | 2021 |
| J Sheehy Haulage Limited | Transport & logistics | 2015 |
| J&B Nails & Spa Limited | Other services | 2025 |
| J&G 86 Property Investment Limited | Real estate | 2026 |
| J&G Tiling Ltd | Construction | 2021 |
| J'Noir Entertainment Ltd | Arts & recreation | 2001 |
| J'Noir Presents CIC | Arts & recreation | 2023 |
| J-One Twickenham Limited | Other services | 2023 |
| J.P. Stephens & SON (London) Limited | Construction | 1987 |
| J3W Limited | Information & communication | 2022 |
| JA Sports Coaching Ltd | Arts & recreation | 2014 |
| JAA Consulting Limited | Real estate | 2025 |
| JAA Labs Limited | Information & communication | 2026 |
| Jaar Ltd | Information & communication | 2017 |
| Jacek Partridge Limited | Professional & scientific | 2018 |
| Jack Thomson Limited | Wholesale & retail | 2012 |
| Jacks of Blackheath Plumbing and Heating Limited | Construction | 2026 |
| Jackson Wise Builders Ltd | Construction | 2013 |
| Jacky Levy Costume Designer Limited | Information & communication | 2025 |
| Jadacia Management Limited | Real estate | 2012 |
| Jadd Properties Limited | Construction | 2015 |
| Jadeed Properties Ltd | Real estate | 2024 |
| JAG Food and Wine Limited | Wholesale & retail | 2014 |
| Jaimebike Ltd | Wholesale & retail | 2020 |
| Jain (Europe) Limited | Wholesale & retail | 1996 |
| Jain International Foods Limited | Manufacturing | 2009 |
| Jaison Singh Limited | Wholesale & retail | 2019 |
| Jake Ferrari Beach Carpentry Ltd | Manufacturing | 2023 |
| James Butler Social Work Limited | Health & social care | 2015 |
| James Chard Limited | Professional & scientific | 2019 |
| Jameson Finance Limited | Information & communication | 2019 |
| Janan Biohealth Ltd | Wholesale & retail | 2025 |
| Jandi Enterprises Ltd | Information & communication | 2013 |
| Jane Greene Limited | Extraterritorial | 2014 |
| Janus International Europe Holdings Ltd | Financial services | 2018 |
| Janus International Europe Limited | Construction | 1996 |
| Janushka Limited | Wholesale & retail | 2011 |
| Jassar Ltd | Professional & scientific | 2022 |
| Jawbone Brewing Ltd | Manufacturing | 2019 |
| JAY Phillips Partnership Ltd | Transport & logistics | 2012 |
| Jays Food and Wine Ltd | Wholesale & retail | 2026 |
| JBS Applied A.I & Robotics Research Ltd | Wholesale & retail | 2020 |
| JBS Roofing Services Ltd | Construction | 2021 |
| JBW Accounting Services Limited | Professional & scientific | 2014 |