| THE PV Academy Ltd | Information & communication | 2026 |
| THE Pygmy Hippo Foundation | Arts & recreation | 2011 |
| THE Quadrant Management Company (Rickmansworth) Limited | Households | 2005 |
| THE Reach Free School Limited | Education | 2012 |
| THE Removals Taxi Limited | Transport & logistics | 2017 |
| THE Retail Kitchen Limited | Professional & scientific | 2018 |
| THE Rickmansworth Clinic Limited | Health & social care | 2026 |
| THE Ripon Operatic Summer School | Education | 1997 |
| THE Royal Masonic School for Girls | Education | 1977 |
| THE Sleepwear Company Limited | Wholesale & retail | 2014 |
| THE Small Luxuries Trading Company Limited | Manufacturing | 2014 |
| THE Sound Sniffer Limited | Arts & recreation | 2024 |
| THE Surveyor’S Surveyor Ltd | Professional & scientific | 2024 |
| THE Swillett Portfolio Limited | Professional & scientific | 2000 |
| THE Tea Shack Limited | Hospitality | 2021 |
| THE Tenancy Team Ltd | Real estate | 2024 |
| THE Top House Croxley Green Ltd | Hospitality | 2025 |
| THE Treasury Chest Limited | Professional & scientific | 2009 |
| THE Troy Wharf Trust Limited | Education | 2000 |
| THE Vertex Physio Group Ltd | Health & social care | 2023 |
| THE Willows Estate Limited | Real estate | 1987 |
| Theatrery Ltd | Hospitality | 2020 |
| Theinstallation.Co.Uk Limited | Construction | 2001 |
| Themed Amusements Ltd | Arts & recreation | 2017 |
| Theovenguys Limited | Administrative & support | 2025 |
| Thermaspace Limited | Manufacturing | 2021 |
| Think Games Advisory Limited | Professional & scientific | 2018 |
| Thinkeractivity Ltd | Administrative & support | 2013 |
| Thirstee Business (Southern) Limited | Administrative & support | 2009 |
| Thirstee Limited | Administrative & support | 2014 |
| Thirty Two Consulting Limited | Wholesale & retail | 2020 |
| Thomas Mcelwee Ltd | Professional & scientific | 2026 |
| Thompson Technology Ltd | Information & communication | 2010 |
| Thornton Consultants Ltd | Professional & scientific | 2019 |
| Threadneedle Commercial Property Ltd | Real estate | 2023 |
| Threadneedle Propco Ltd | Real estate | 2018 |
| Threadneedle Propco SW Limited | Real estate | 2024 |
| Threadneedle Property Holdings Ltd | Financial services | 2023 |
| Threadneedle Recruitment Ltd | Financial services | 2023 |
| Three Quoins Ltd. | Real estate | 1999 |
| Three Rivers Chamber of Trade & Commerce CIC | Other services | 2016 |
| Three Rivers Commercial Services Limited | Administrative & support | 2017 |
| Three Rivers Consulting Ltd | Professional & scientific | 2014 |
| Three Rivers Museum Trust | Arts & recreation | 1994 |
| Three Rivers Stone Ltd | Wholesale & retail | 2022 |
| Threebee Media Ltd | Information & communication | 2015 |
| Threedumb Theatre Ltd | Arts & recreation | 2023 |
| Threshold Estate Ltd | Real estate | 2019 |
| Thuwa Property Services Ltd | Real estate | 2025 |
| Thynq Technologies Limited | Information & communication | 2025 |
| Ticketgrange Limited | Arts & recreation | 1983 |
| Tide 2 Tide Properties Limited | Construction | 2020 |
| Tigerpixel Limited | Information & communication | 2013 |
| Tileopedia UK Ltd | Construction | 2022 |
| Tilia Advisory Ltd | Professional & scientific | 2026 |
| Timberly Group Ltd | Construction | 2026 |
| Timeline London Limited | Construction | 2010 |
| Timipix Limited | Professional & scientific | 2016 |
| Timlana Consultancy Services Ltd | Professional & scientific | 2014 |
| Tipmac Properties Ltd | Real estate | 2022 |
| Tippen Group Limited | Real estate | 2026 |
| TIS Holidays Limited | Administrative & support | 2014 |
| Titanas Limited | Wholesale & retail | 2019 |
| Titanium Consultancy (IT) Limited | Information & communication | 2007 |
| Tiwi Group Ltd | Financial services | 2021 |
| TLA Properties Ltd | Households | 2022 |
| TLC Operations Ltd | Administrative & support | 2026 |
| TLC Prep Ltd | Professional & scientific | 2025 |
| Tlfb Production Services Ltd | Administrative & support | 2019 |
| TLS Energy Limited | Professional & scientific | 2001 |
| TLX Cargo Limited | Manufacturing | 2020 |
| TLX Holdings Limited | Manufacturing | 2020 |
| TLX Insulation Limited | Manufacturing | 2013 |
| TLX Manufacturing Limited | Manufacturing | 2014 |
| TM Corporate Services Ltd | Professional & scientific | 2016 |
| TNM Quality Homes Limited | Real estate | 2024 |
| Toby Brand Agriculture Ltd | Agriculture & fishing | 2024 |
| Tolia House Residents RTM Company Limited | Other services | 2012 |
| Tompkins Jewellers Ltd | Wholesale & retail | 2018 |
| Top-Teks Investments Limited | Administrative & support | 2008 |
| Top-Teks Limited | Wholesale & retail | 1989 |
| Topologie UK Limited | Wholesale & retail | 2023 |
| Topteam Electrical Limited | Construction | 2013 |
| Tort Limited | Manufacturing | 2025 |
| Total Construction Limited | Construction | 1999 |
| Total Contractors Limited | Construction | 2025 |
| Total Excellence Limited | Professional & scientific | 2018 |
| Total Shading Solutions Limited | Construction | 2025 |
| Total Solutions Consultants Ltd | Professional & scientific | 2026 |
| Touchable Limited | Manufacturing | 2026 |
| Touchstone Mastertechs Limited | Information & communication | 2017 |
| Touchstone Properties (2000) Ltd. | Real estate | 2000 |
| Tpmg Group Ltd | Administrative & support | 2023 |
| Tpmg Group Services Ltd | Administrative & support | 2026 |
| Track Audio Ltd | Manufacturing | 2010 |
| Track Distribution Limited | Wholesale & retail | 2025 |
| Trade Bank CIC | Construction | 2022 |
| Traditional Foods Limited | Manufacturing | 1991 |
| Trafalgar Bury ST Limited | Real estate | 2018 |
| Trafalgar Development (UK) Limited | Construction | 2005 |