| Top Employers Research LP | | 2006 |
| Top HAT Marquees Limited | Administrative & support | 2013 |
| Top Place Preschool Limited | Education | 2019 |
| Top Priority Clothing Ltd | Wholesale & retail | 2024 |
| Top RB Limited | Construction | 2017 |
| Top Thread Upholstery Ltd | Other services | 2025 |
| Topbrook Developments Limited | Professional & scientific | 1994 |
| Topcar Auto Repairs Ltd | Wholesale & retail | 2020 |
| Topi Construction Ltd | Construction | 2025 |
| Toplevel Car Care Ltd | Wholesale & retail | 2020 |
| Tops Pizza (Hemel Hempstead) Ltd | Hospitality | 2021 |
| Topuzi Contractors Ltd | Construction | 2022 |
| TOR Hill Management & Maintenance Limited | Real estate | 2006 |
| Tornado Storage Solutions (Holdings) Ltd | Financial services | 2024 |
| Tornado Storage Solutions Limited | Manufacturing | 2003 |
| Toroverde Ltd | Manufacturing | 2018 |
| Torque Italia Limited | Wholesale & retail | 2020 |
| Torricel Limited | Information & communication | 2018 |
| Torsional Ltd | Professional & scientific | 2018 |
| Tosnetwork Ltd | Information & communication | 2025 |
| Tostrive Construction Limited | Construction | 2019 |
| Tostrive Ltd | Construction | 2014 |
| Total Ability Limited | Construction | 2012 |
| Total Equipment Ltd | Wholesale & retail | 2025 |
| Total Flooring 23 Ltd | Wholesale & retail | 2023 |
| Total Flooring Installations Ltd | Administrative & support | 2012 |
| Total Group Ltd | Construction | 2026 |
| Total Property Care Limited | Administrative & support | 1992 |
| Total Property Services London Ltd | Construction | 2025 |
| Total Render Limited | Construction | 2016 |
| Total Roofing (Contractors) Ltd | Construction | 2025 |
| Total Support Glass and Glazing Limited | Professional & scientific | 2010 |
| Total Support Training Limited | Administrative & support | 2006 |
| Total Support Vocational Academy Limited | Administrative & support | 2017 |
| Totalfix Solutions Ltd | Construction | 2025 |
| Totally Aw:Some Media Limited | Information & communication | 2018 |
| Tots Togz Ltd | Wholesale & retail | 2020 |
| Tottenham Management Holding Ltd | Hospitality | 2025 |
| Tottenham Management Ltd | Hospitality | 2025 |
| Totz Town Ltd | Health & social care | 2023 |
| Touch Wood Creations Ltd | Construction | 2021 |
| Touchline Warrior Limited | Wholesale & retail | 2019 |
| Townhouse Plumbing & Heating Ltd | Construction | 2017 |
| Townsend Carpentry Ltd | Construction | 2022 |
| TP Education Ltd | Education | 2025 |
| TR Howlings Limited | Real estate | 2024 |
| Trace One Limited | Information & communication | 1997 |
| Tracey Newson Consultancy Limited | Health & social care | 2012 |
| Track Record Limited | Professional & scientific | 2019 |
| TRACKIT247 Limited | Other services | 2009 |
| Trade Roofing Supplies Ltd | Wholesale & retail | 2021 |
| Trade Soloutions Limited | Administrative & support | 2018 |
| Trade Tyre Services Limited | | 1976 |
| Traders Mate Limited | Other services | 2016 |
| Trades Desk Ltd | Wholesale & retail | 2016 |
| Tradetech Analytics Ltd | Information & communication | 2023 |
| Trailer Resources Limited | Wholesale & retail | 1994 |
| Train & Sustain Online Coaching Ltd | Other services | 2023 |
| Tramontana Holding LLP | | 2025 |
| Tranby Fields Management Company Limited | Households | 2016 |
| Trans Two U Ltd | Wholesale & retail | 2023 |
| Transformation Strategies Ltd | Professional & scientific | 2022 |
| Transformix Ltd | Professional & scientific | 2024 |
| Transformosys Limited | Professional & scientific | 2010 |
| Transient Systems Limited | Information & communication | 2025 |
| Transonics Limited | Manufacturing | 1976 |
| Transport and Store Ltd | Transport & logistics | 2024 |
| Transport Auto Ltd | Transport & logistics | 2025 |
| Transport CM Limited | Professional & scientific | 2020 |
| Transprime Ltd | Transport & logistics | 2023 |
| Tratt Limited | Construction | 2014 |
| Trauma & Eating Disorder Specialists (Teds) Ltd | Health & social care | 2020 |
| Travelgraph Limited | Wholesale & retail | 2014 |
| Trayne Ltd | Wholesale & retail | 2024 |
| Tree Bros Ltd | Administrative & support | 2023 |
| Tree Tone Limited | Professional & scientific | 2015 |
| Treebyte Systems Ltd | Information & communication | 2025 |
| Treehouse E-Commerce Group UK Ltd | Real estate | 2021 |
| Treelife Limited | Agriculture & fishing | 2012 |
| Trell Ltd | Information & communication | 2025 |
| Trellis Care Limited | Health & social care | 2021 |
| Trend & Thomas Surveyors Limited | Real estate | 2004 |
| Trentham Fields Management Company NO 1 Limited | Real estate | 2023 |
| Trentham Fields Management Company NO 2 Limited | Real estate | 2023 |
| Trentham Fields Management Company NO 3 Limited | Real estate | 2023 |
| Trentham Fields Management Company NO 4 Limited | Real estate | 2023 |
| Trentham Fields Management Company NO 5 Limited | Real estate | 2023 |
| Tresilian Limited | Real estate | 2017 |
| TRH Electrical Solutions Ltd | Construction | 2021 |
| Tri-Scan Limited | Extraterritorial | 1974 |
| Triangle Debt Resolution Limited | Administrative & support | 2026 |
| Triangle Infotech Limited | Information & communication | 1992 |
| Triangle Software Limited | Information & communication | 2010 |
| Tribune Designs Limited | Professional & scientific | 1986 |
| Tridan Business Concepts Limited | Education | 2014 |
| Trieste Group Holdings Ltd | Financial services | 2024 |
| Trieste Group LLP | | 2012 |
| Trieste Group One Ltd | Manufacturing | 2012 |
| Triforce Employment Solutions Ltd | Administrative & support | 2024 |
| Trimlock Services Limited | Construction | 2013 |