| Student Experience Consultancy Ltd | Administrative & support | 2019 |
| Stupal Ltd | Real estate | 2021 |
| Success Coaching & Development Limited | Administrative & support | 2008 |
| Sureplan Technical Limited | Construction | 2014 |
| Swiftloop Associates Ltd | Information & communication | 2026 |
| Swim Plus POP UP Pools Limited | Education | 2024 |
| Syl&Taylor Ltd | Agriculture & fishing | 2025 |
| T.S.F. (Heating) Limited | | 1974 |
| TAJ Mahal Welwyn Takeout Service Limited | Wholesale & retail | 2020 |
| Talent and Tale Ltd | Professional & scientific | 2026 |
| Talent Decentral Ltd | Professional & scientific | 2022 |
| Tamblin Ltd | Real estate | 2021 |
| Tanzit Ltd | Extraterritorial | 2012 |
| Target Drainage & Plumbing Ltd | Construction | 2022 |
| Tcbuilders Ltd | Manufacturing | 2020 |
| TCF Property Investments Limited | Real estate | 2021 |
| Team 2 Management Ltd | Professional & scientific | 2023 |
| Teamwize Ltd | Extraterritorial | 2020 |
| TED Kavanagh Limited | Administrative & support | 2024 |
| TEN Wendover Lodge (Management Company) Limited | Households | 2012 |
| Tennis Breaks Ltd | Administrative & support | 2012 |
| Terrain Holdings Limited | Financial services | 2026 |
| Terrain Surveys Limited | Professional & scientific | 2004 |
| Tewin Bury Farm Hotel LLP | | 2016 |
| Tewin Bury Farm Limited | Hospitality | 2003 |
| Tewin Bury Farm Services Limited | Administrative & support | 2020 |
| Tewin Joinery Ltd | Manufacturing | 2025 |
| Tewin Pavilion Community Interest Company | Arts & recreation | 2010 |
| Tewin Water Farm Management Company Limited | Real estate | 2004 |
| Tewin Wood Roads Limited | Households | 1953 |
| THE Access Guardian Limited | Administrative & support | 2025 |
| THE Accounts Factory Limited | Professional & scientific | 2005 |
| THE Artist Surfaces Company (London) Ltd | Wholesale & retail | 2019 |
| THE Association of Governing Bodies of Independent Schools | Education | 2004 |
| THE Ayot ST Lawrence Old Church Preservation Trust | Arts & recreation | 1998 |
| THE Barnes Physiotherapy Practice Ltd | Health & social care | 2011 |
| THE Bee-Primrose Company Limited | Professional & scientific | 1996 |
| THE Birth and Baby Company Ltd | Education | 2024 |
| THE Cleanroom Company Ltd | Construction | 2006 |
| THE Crooked Billet (Ware) Ltd | Hospitality | 2019 |
| THE Elliott Partnership Limited | Professional & scientific | 2006 |
| THE Hellenic Deli (London) Ltd | Wholesale & retail | 2019 |
| THE Hoofsmiths Ltd | Professional & scientific | 2025 |
| THE Jetram Partnership Limited | Professional & scientific | 2008 |
| THE Key Worker Group Limited | Financial services | 2020 |
| THE Leech Group Ltd | Construction | 2017 |
| THE Little Sweet Company Ltd | Wholesale & retail | 2020 |
| THE Moyes Clinic Ltd | Health & social care | 2024 |
| THE Nest Welwyn Ltd | Other services | 2025 |
| THE Neurodivergent Leisure Group CIC | Education | 2025 |
| THE Private Wine Collection Ltd | Wholesale & retail | 2023 |
| THE Red House Residents Management Ltd | Households | 2020 |
| THE Satori Partnership Limited | Professional & scientific | 2024 |
| THE Stitch and Craft Company Limited | Wholesale & retail | 2023 |
| THE Walled Garden Residents Company Limited | Real estate | 2004 |
| THE Wardrobe That Works Ltd | Real estate | 2002 |
| THE Warren Management Company Limited | Households | 1997 |
| THE Whole KIT Ltd | Professional & scientific | 2017 |
| Think Outside the Box Ltd | Professional & scientific | 2008 |
| Thirty Acre Farm Limited | Agriculture & fishing | 2026 |
| Three Counties Youth Ballet Limited | Education | 2009 |
| Thrust Consulting Limited | Information & communication | 2007 |
| Thurd Limited | Information & communication | 2024 |
| Tinsley Consulting Limited | Professional & scientific | 2025 |
| Titan House Ltd | Financial services | 2009 |
| TLC Moves Limited | Other services | 2017 |
| TM Welwyn Limited | Hospitality | 2018 |
| TOM Hill Works Ltd | Arts & recreation | 2023 |
| TOM O'Bedlam Limited | Real estate | 2014 |
| Top Class Wine Ltd | Wholesale & retail | 2019 |
| Total Maintenance UK Limited | Construction | 1993 |
| Touchwood Bespoke Furniture Ltd | Manufacturing | 2023 |
| TR JB Scaffolding Ltd | Construction | 2025 |
| Trade My Clutter Limited | Other services | 2015 |
| Treme Design LLP | | 2011 |
| Trevena Gardens (Management Company) Limited | Other services | 2010 |
| Trimur Investments Ltd | Real estate | 2026 |
| Trini Soul Ltd | Hospitality | 2025 |
| Trinity Publishing Ltd | Information & communication | 2026 |
| Tsuwa Ltd | Information & communication | 2023 |
| Turn 2 Ltd | Professional & scientific | 2006 |
| Tutor ME Limited | Education | 2011 |
| Ummah FC Ltd | Arts & recreation | 2025 |
| Uniboosters Ltd | Other services | 2024 |
| United Coffee Limited | Wholesale & retail | 2003 |
| UP Design (Import) Limited | Wholesale & retail | 1992 |
| V a General Service Ltd | Transport & logistics | 2010 |
| V10DETAILING Ltd | Manufacturing | 2020 |
| Vacations by Design Limited | Administrative & support | 1995 |
| Valdez Gymnastics Limited | Arts & recreation | 2014 |
| Valenti Property Limited | Real estate | 2021 |
| Valerie Hawkes Limited | Information & communication | 1993 |
| Variation Holdings Limited | Transport & logistics | 2006 |
| Variation Holdings UK Limited | Transport & logistics | 2015 |
| VET View Limited | Professional & scientific | 2010 |
| VG Wine Library Limited | Real estate | 2003 |
| Viewpoint 3D Limited | Manufacturing | 2009 |
| Vikriya Pharmaceuticals Ltd | Wholesale & retail | 2024 |
| Vinu Investments Ltd | Wholesale & retail | 2024 |
| Virtual Reality Multimedia Limited | Wholesale & retail | 2018 |