| Union Court Property Management Limited | Households | 1999 |
| Union Fibre Pipes (Great Britain) Limited | | 1956 |
| Union GAP LLP | | 2017 |
| Union Mondiale DES Leaders Chretiens Pour LE Reveil Spirituel C.I.C. | Other services | 2024 |
| UNION3MEDIA Ltd | Administrative & support | 2018 |
| Uniprof LLP | | 2018 |
| Uniq Group Ltd | Construction | 2025 |
| Unique EYE Care Ltd | Health & social care | 2006 |
| Unique F&D Ltd | Wholesale & retail | 2022 |
| Unique Gems & Jewels Ltd | Wholesale & retail | 2020 |
| Unique Metal Jewelry Ltd | Wholesale & retail | 2023 |
| Unit 2 (Rickmansworth) Limited | Real estate | 2013 |
| United Associates Limited | Information & communication | 2012 |
| United Kingdom Deliveries and Removals Limited | Administrative & support | 2017 |
| United Vision Technology Ltd | Information & communication | 2024 |
| Unitrade Infotech Limited | Information & communication | 2016 |
| Units of Sound Ltd | Information & communication | 1997 |
| Unity Community Events CIC | Other services | 2026 |
| Unity Estate Management Company Limited | Real estate | 2021 |
| Unity LSS Limited | Professional & scientific | 2010 |
| Unity Recruitment Group Limited | Administrative & support | 2018 |
| Unity Recruitment Ltd | Administrative & support | 2007 |
| Unityware Solutions Ltd | Manufacturing | 2023 |
| Universal Accountancy Ltd | Professional & scientific | 2014 |
| Universal Clean Ltd | Administrative & support | 2023 |
| Universal Engine Power Limited | Wholesale & retail | 1986 |
| Universal I Facility Services Limited | Administrative & support | 2019 |
| Universal Protection Limited | Construction | 2009 |
| Universal Resorts (UK) Limited | Extraterritorial | 1999 |
| Universal Roofing (UK) Ltd | Construction | 2014 |
| Universal Toiletries Corporation Limited | Manufacturing | 1999 |
| Universal Unity Limited | Wholesale & retail | 2011 |
| Unlimited Wellbeing Psychology Services Ltd | Health & social care | 2018 |
| Unlimiting Factor Ltd | Professional & scientific | 2020 |
| Unprescribed Sips Ltd | Wholesale & retail | 2026 |
| Unshakable Limited | Wholesale & retail | 2025 |
| Untamed Notions Limited | Information & communication | 2024 |
| UNV Accountants Ltd | Professional & scientific | 2019 |
| Unver Holding Ltd | Financial services | 2021 |
| UOS Events Ltd | Arts & recreation | 2024 |
| UP Business Management Limited | Professional & scientific | 2019 |
| UP Central Investments Ltd | Financial services | 2023 |
| UP DJS Homes Limited | Real estate | 2026 |
| UP Osas Carpentry and Joinery Limited | Manufacturing | 2026 |
| Up-Turn Logistics Ltd | Transport & logistics | 2022 |
| Upasna Ltd | Construction | 2020 |
| Upharm Ltd | Education | 2022 |
| Upholstery by Design Limited | Manufacturing | 2009 |
| Uponor Limited | Wholesale & retail | 1997 |
| Upoption Limited | Information & communication | 2019 |
| Upper Street Property Consultants Limited | Real estate | 2022 |
| Uprest Property Management Limited | Real estate | 1973 |
| Upscale Solutions Ltd | Information & communication | 2013 |
| Upside Investors Ltd | Real estate | 2024 |
| Uptown Dogs Ltd | Other services | 2014 |
| Uptown Interiors Ltd | Construction | 2021 |
| Upvanta Ltd | Professional & scientific | 2022 |
| Ural Consulting Ltd | Administrative & support | 2011 |
| Urantub Ltd | Construction | 2026 |
| Urban CS Construction Ltd | Construction | 2017 |
| Urban Design & Construction Ltd | Construction | 2016 |
| Urban Economics International Ltd | Professional & scientific | 2021 |
| Urban Gallery Ltd | Wholesale & retail | 2015 |
| Urban Gorny Limited | Transport & logistics | 2016 |
| Urban Progress Ttpro Ltd | Arts & recreation | 2006 |
| Urban Solutions Limited | Construction | 2023 |
| Urban Table Tennis Club | Arts & recreation | 2012 |
| Urbana Property Group Ltd | Extraterritorial | 2009 |
| Urbancrazy (Mobile) Limited | Arts & recreation | 2002 |
| Urbane Group Limited | Real estate | 2013 |
| Urbanhealth Support Ltd | Information & communication | 2023 |
| Urbanspin TT Ltd | Arts & recreation | 2013 |
| Urbent Limited | Administrative & support | 2025 |
| Urec Ltd | Education | 2019 |
| Urjai Investments Limited | Real estate | 2015 |
| Urmi News Limited | Wholesale & retail | 2019 |
| Urmi Properties Ltd | Real estate | 2025 |
| Urthly Ltd | Manufacturing | 2022 |
| Uslot Limited | Information & communication | 2013 |
| Usman a Limited | Wholesale & retail | 2023 |
| Usman Supermart Ltd | Wholesale & retail | 2026 |
| USN Investments Ltd | Real estate | 2019 |
| Usui Reiki School Limited | Health & social care | 2023 |
| Util Investment Company Limited | Real estate | 2016 |
| Utili-Trees Limited | Energy | 2023 |
| Utilities Investment Management Limited | Professional & scientific | 2002 |
| Utilities U.K. Limited | Information & communication | 1995 |
| Utopia Housing Holdings Ltd | Real estate | 2018 |
| Utopia Supported Housing Ltd | Health & social care | 2016 |
| UV Tech Ltd | Information & communication | 2015 |
| UW Consultancy Limited | Financial services | 2025 |
| UW Consulting Ltd | Professional & scientific | 2016 |
| Uxbridge Hire Company Limited | Real estate | 1963 |
| Uxspecifics Ltd | Information & communication | 2012 |
| UZA Tech Store Ltd | Other services | 2019 |
| V & S (NO 15) Limited | Real estate | 1989 |
| V B Johnson (Consultancy) Limited | Construction | 2016 |
| V B Johnson LLP | | 2004 |
| V H Investments Limited | Real estate | 2016 |
| V Liburd Consulting Ltd | Information & communication | 2018 |