| Under Budgett Limited | Professional & scientific | 2017 |
| Under the Thinking Tree C.I.C. | Health & social care | 2020 |
| Underdogs Norfolk Ltd | Hospitality | 2019 |
| Undersea Cable Consultancy Limited | Professional & scientific | 2017 |
| Undertow CIC | Arts & recreation | 2024 |
| Underwater Art Limited | Professional & scientific | 2012 |
| Underwoods Farms Limited | Extraterritorial | 1959 |
| Undo Limited | Information & communication | 2005 |
| Unfolding Retreats Ltd | Other services | 2025 |
| Ungi Limited | Manufacturing | 2022 |
| Ungureanu Aviatie Ltd | Manufacturing | 2023 |
| Ungureanu Catalin.Smw Ltd | Manufacturing | 2023 |
| UNI Barbers Cambridge Limited | Other services | 2023 |
| UNI Gateway UK Limited | Education | 2025 |
| UNI TAC Catering UK Ltd | Hospitality | 2022 |
| Unicorn Universal Ltd | Wholesale & retail | 2021 |
| Unidea Limited | Education | 2001 |
| Unified Home Care Ltd | Administrative & support | 2021 |
| Unified Needs Ltd | Education | 2025 |
| Uniformity Investment Center Co., Ltd | Extraterritorial | 2015 |
| Unify Schools Ltd | Professional & scientific | 2020 |
| Unifying Services Ltd | Information & communication | 2023 |
| Unihive Education Ltd | Education | 2024 |
| Unik Consultancy Ltd | Professional & scientific | 2026 |
| Unikko Ltd | Professional & scientific | 2018 |
| Uniolordy International Ltd | Wholesale & retail | 2022 |
| Union Wisdom (UK) Ltd | Extraterritorial | 2019 |
| Unipath Advisors Limited | Education | 2026 |
| Uniplus Technologies Ltd | Information & communication | 2026 |
| Unipre Trading Co., Ltd | Extraterritorial | 2024 |
| Uniq Health Ltd | Professional & scientific | 2017 |
| Unique Cambridge Ltd | Manufacturing | 2025 |
| Unique Chemical Limited | Professional & scientific | 2008 |
| Unique Essence Ltd | Manufacturing | 2025 |
| Unique Lettings & Property Management Ltd | Real estate | 2015 |
| Unique Speed Ltd | Wholesale & retail | 2025 |
| Unique Speed Technology Co., Ltd | Manufacturing | 2025 |
| Uniquely Cricklewood Management Company Ltd | Real estate | 2022 |
| Unisky Sign Industrial Co., Ltd | Wholesale & retail | 2018 |
| Unit Properties Ltd | Real estate | 2019 |
| Unite Green Ltd | Extraterritorial | 2013 |
| United BEE Ltd | Wholesale & retail | 2025 |
| United Digital Asset Exchange Ltd | Financial services | 2026 |
| United Kingdom Antarctic Heritage Trust | Arts & recreation | 2015 |
| United Kingdom Elman Education Group Ltd | Education | 2024 |
| United Kingdom Keruite Sealing Technology Co Ltd | Manufacturing | 2020 |
| United Kingdom Play Holdings Ltd | Financial services | 2025 |
| United Kingdom Play Ltd | Wholesale & retail | 2025 |
| United Nations Starlink Development Committee | Wholesale & retail | 2023 |
| United Parents Ltd | Information & communication | 2025 |
| United Printing Limited | Extraterritorial | 2011 |
| Unitex Group Ltd | Manufacturing | 2023 |
| Unitouruk Ltd | Administrative & support | 2026 |
| Unity Padel Ltd | Arts & recreation | 2024 |
| Unityy Trade Ltd | Wholesale & retail | 2025 |
| Universal Biologicals (Cambridge) Limited | Wholesale & retail | 1980 |
| Universal Biologicals Limited | Administrative & support | 2003 |
| Universal Composite Doors Ltd | Manufacturing | 2017 |
| Universal Download Ltd | Manufacturing | 2024 |
| Universal Fencing Limited | Wholesale & retail | 1998 |
| Universal Garden Supplies-Ridgeons Limited | Extraterritorial | 1945 |
| Universal Groundworks Ltd | Construction | 2022 |
| Universal Healing Limited | Education | 2020 |
| Universal Land Corporation Limited | Real estate | 2024 |
| University of Cambridge Investment Management Limited | Financial services | 2009 |
| Unknownworld-Onlinestores Ltd | Wholesale & retail | 2026 |
| Unlicensed Film Ltd | Information & communication | 2023 |
| Unlikely Artificial Intelligence Limited | Information & communication | 2018 |
| Unlock Enterprises Ltd | Professional & scientific | 2025 |
| Unrealise Ltd | Professional & scientific | 2026 |
| Unrecruit Limited | Professional & scientific | 2025 |
| Unveil Lifting Club Ltd | Wholesale & retail | 2024 |
| Unwin Developments Ltd | Construction | 2025 |
| Unwin Lettings Ltd | Real estate | 2025 |
| UP Aero Ltd | Manufacturing | 2026 |
| Upair Ltd | Manufacturing | 2025 |
| Uplift Financial Services Limited | Financial services | 2024 |
| Upper Park Road (Camberley) Management Company Limited | Households | 2004 |
| Upperfield | Financial services | 2022 |
| Upstay Ltd | Hospitality | 2017 |
| Upton's of Saffron Walden Limited | Professional & scientific | 1998 |
| UQG Optics Limited | Administrative & support | 2020 |
| UR Ride Ltd | Transport & logistics | 2024 |
| Urban Agenda Limited | Construction | 2003 |
| Urban Brew House Ltd | Wholesale & retail | 2025 |
| Urban Caffeine Ltd | Hospitality | 2026 |
| Urban Car Company Ltd | Wholesale & retail | 2008 |
| Urban Chariot Limited | Hospitality | 2024 |
| Urban Freight Limited | Transport & logistics | 2025 |
| Urban Garden Buildings Limited | Construction | 2019 |
| Urban Pearl Limited | Real estate | 2019 |
| Urban Spoon Limited | Hospitality | 2026 |
| Urban Summit Construction Limited | Construction | 2013 |
| Urban Summit Development Limited | Construction | 2013 |
| Urban Summit Group Limited | Construction | 2013 |
| Urban Summit Holdings Limited | Financial services | 2021 |
| Urban Summit Homes Limited | Construction | 2018 |
| Urban Summit Plant Limited | Administrative & support | 2018 |
| Urban Summit Solutions Limited | Construction | 2019 |
| Urbanska Care Limited | Health & social care | 2017 |