| Isis (Phase 1) Cardiff Management Company Limited | Households | 2004 |
| Isis (Phase 2) Cardiff Management Company Limited | Households | 2005 |
| ISL Online Ltd. | Information & communication | 2008 |
| Island Care and Support Ltd | Administrative & support | 2016 |
| Island Homecare Ltd | Health & social care | 2021 |
| Island Medz Ltd | Hospitality | 2026 |
| Isobel Bowles Property Limited | Real estate | 2024 |
| Isom Group Limited | Wholesale & retail | 2020 |
| Ison Trade Ltd | Transport & logistics | 2019 |
| ISP International Space Propulsion Ltd | Professional & scientific | 2001 |
| Istanbul Barber Marlborough Ltd | Other services | 2022 |
| Istanbul Market Ltd | Wholesale & retail | 2025 |
| IT (Tech) Cleaning Limited | Information & communication | 2008 |
| IT Analysis & Training Ltd | Information & communication | 2019 |
| IT Cleaning Limited | Administrative & support | 2002 |
| IT Probe Ltd | Information & communication | 2022 |
| Italian Bar and Restaurant Limited | Hospitality | 2024 |
| Italian Foodexpo Ltd | Wholesale & retail | 2015 |
| Itam Forum Ltd | Other services | 2019 |
| Itam Review Learning Limited | Information & communication | 2021 |
| Itandcyber Security Ltd | Information & communication | 2026 |
| Itci Ltd | Transport & logistics | 2015 |
| Itel Electronics Limited | Manufacturing | 1997 |
| Itest Limited | Health & social care | 2022 |
| Itex Food Ltd | Wholesale & retail | 2023 |
| Itpam Ltd | Professional & scientific | 2018 |
| Itrf (Global) Limited | Information & communication | 2016 |
| ITS Automation & Servicing Ltd | Professional & scientific | 2023 |
| ITS Platinum NOT Grey Ltd | Other services | 2017 |
| Itspr Global Ltd | Professional & scientific | 2016 |
| IVA Transfers Ltd | Transport & logistics | 2025 |
| Ivar Innovations Limited | Professional & scientific | 2021 |
| Iviesano & Shinehealth Limited | Health & social care | 2025 |
| IVY Rose Tools Ltd | Wholesale & retail | 2019 |
| Ivy's Cleaning Company Ltd | Administrative & support | 2026 |
| IZ Digital Ltd | Information & communication | 2020 |
| Izikimag Healthcare Limited | Health & social care | 2018 |
| Izukodi Healthcare Limited | Health & social care | 2024 |
| Izyaan Limited | Wholesale & retail | 2026 |
| J & D Decorating Services Ltd | Construction | 2022 |
| J & F Solutions Ltd | Administrative & support | 2024 |
| J & F Turner Limited | Hospitality | 2010 |
| J & K Brodie Holdings Ltd | Financial services | 2024 |
| J & L Groundworks (Swindon) Ltd | Construction | 2016 |
| J & M Motor Services Limited | Transport & logistics | 2018 |
| J & S Electrical Limited | Construction | 2007 |
| J & S Properties (Swindon) Limited | Real estate | 2009 |
| J & S Roofing and Bricklaying Contractors Limited | Construction | 2007 |
| J Auto Service Limited | Wholesale & retail | 2021 |
| J B UK Care Services Limited | Administrative & support | 2020 |
| J C Scuba Ltd | Arts & recreation | 2011 |
| J Carpets Limited | Other services | 2014 |
| J D B Carpentry Limited | Construction | 2008 |
| J DE Souza Building Ltd | Other services | 2025 |
| J Dilley Book Keeping Limited | Professional & scientific | 2021 |
| J Dixon Hospitality Group Limited | Hospitality | 2025 |
| J E Palmer Engineering Consultancy Limited | Professional & scientific | 2018 |
| J Farren Ltd | Construction | 2013 |
| J Flooring Limited | Construction | 2019 |
| J G Restorations Limited | Construction | 2009 |
| J Gantlett Ltd | Real estate | 2011 |
| J Gough Building Services Ltd | Construction | 2014 |
| J H B Construction Ltd | Administrative & support | 2017 |
| J H Fitting Services Limited | Construction | 2012 |
| J Holder Limited | Professional & scientific | 2016 |
| J J Financial Services Limited | Financial services | 2006 |
| J J Garage Limited | Wholesale & retail | 2024 |
| J Johnson Roofing Ltd | Construction | 2025 |
| J K H Masonry Cleaning Limited | Administrative & support | 2004 |
| J K Roofing Swindon Ltd | Construction | 2023 |
| J KOS Building Service UK Ltd | Construction | 2023 |
| J M L Civil Engineering & Groundworks Limited | Construction | 2021 |
| J N GAS Ltd | Construction | 2021 |
| J P Flanagan & Sons Limited | Construction | 2009 |
| J R Amenities Ltd | Water & waste | 2020 |
| J R Machinery Solutions Ltd | Wholesale & retail | 2026 |
| J Regan Security Services Ltd | Administrative & support | 2012 |
| J Rudman Financial Services Ltd | Financial services | 2025 |
| J S Bath Ltd | Administrative & support | 2016 |
| J S Board Finishings Limited | Construction | 2005 |
| J S Concepts Limited | Construction | 2019 |
| J Smith Plumbing & Heating (Swindon) Ltd | Construction | 2022 |
| J Solutions Services Ltd | Administrative & support | 2024 |
| J Sparling Ltd | Hospitality | 2021 |
| J Viola Financial Services Limited | Financial services | 2017 |
| J W B Contracting Ltd | Hospitality | 2006 |
| J W Consulting Services Limited | Extraterritorial | 2008 |
| J W Duckworth Limited | Professional & scientific | 2010 |
| J&A Elite Ladies Kickboxing Ltd | Arts & recreation | 2024 |
| J&B Real Estate Group Ltd | Real estate | 2025 |
| J&H Foods Ltd | Hospitality | 2021 |
| J&J Hughes Limited | Professional & scientific | 2023 |
| J&J Market Ltd | Wholesale & retail | 2016 |
| J&J Pubs Ltd | Hospitality | 2023 |
| J&M Electrobikes Ltd | Wholesale & retail | 2020 |
| J&M Estates Ltd | Real estate | 2023 |
| J&M Flooring (Holdings) Ltd | Wholesale & retail | 2017 |
| J&M Lets Limited | Construction | 2026 |
| J&O Partners Ltd | Financial services | 2025 |
| J&R Carpentry Services Limited | Construction | 2005 |