| Secret Squirrel Property Limited | Real estate | 2024 |
| Securaco Ltd | Manufacturing | 2020 |
| Securahome Holdings Ltd | Construction | 2022 |
| Secure Events UK Limited | Hospitality | 2018 |
| Secure Freeholds Limited | Real estate | 2024 |
| Secure Heights Ltd | Information & communication | 2023 |
| Secured Alarm Systems Ltd | Administrative & support | 2010 |
| Secured Guard Services Limited | Administrative & support | 2014 |
| Securewills Ltd | Professional & scientific | 2025 |
| Securion Technology Limited | Information & communication | 2004 |
| Security Specialist Services Ltd | Administrative & support | 2018 |
| Seetall Furniture Limited | Manufacturing | 1977 |
| Sefopea Limited | Construction | 2021 |
| Segmed Ltd | Health & social care | 2021 |
| Seibu UK Ltd | Wholesale & retail | 2011 |
| Seicateb Ltd | Education | 2025 |
| Seithfed DON Ltd | Arts & recreation | 2018 |
| Selclene (Battersea) Limited | Other services | 1995 |
| Selclene (Docklands) Limited | Administrative & support | 2000 |
| Selclene (Holdings) Limited | Hospitality | 2004 |
| Selclene (Maida Vale) Limited | Administrative & support | 1994 |
| Selclene (Preston) Limited | Administrative & support | 2003 |
| Selclene (ST. Albans) Ltd | Administrative & support | 2003 |
| Selclene (Surrey) Ltd | Other services | 1997 |
| Selclene Limited | Other services | 1991 |
| Selclene/Supermaids Limited | Other services | 1995 |
| Select Carpets and Vinyls Limited | Construction | 2002 |
| Select Mortgages & Loans Ltd | Financial services | 2023 |
| Select Swansea Radiology Images Ltd | Health & social care | 2021 |
| Self Build Assist Ltd | Construction | 2019 |
| Self Storage for Wales Ltd | Transport & logistics | 2025 |
| Sell Your Caravan Limited | Wholesale & retail | 2020 |
| Sellick Consult Limited | Professional & scientific | 2015 |
| Selminas Ltd | Hospitality | 2017 |
| Selwyn Jones Limited | Real estate | 2006 |
| Selwyn's Limited | Extraterritorial | 2008 |
| Selwyn's Seafoods Limited | Manufacturing | 2008 |
| Selwyns Penclawdd Seafoods Limited | Extraterritorial | 2001 |
| Seminar Components (U.K.) Limited | Manufacturing | 1974 |
| Seminar Components Holdings Limited | Administrative & support | 2013 |
| Semper Elite Wills Ltd | Professional & scientific | 2021 |
| Sensitive Water Solutions Limited | Professional & scientific | 2003 |
| Sentinel Advanced Systems Ltd | Information & communication | 2026 |
| SER Property Investment Limited | Households | 2025 |
| Serafim Service Ltd | Hospitality | 2025 |
| Seren Consult Ltd | Professional & scientific | 2021 |
| Seren Energy Ltd | Professional & scientific | 2003 |
| Seren Fach Limited | Other services | 2010 |
| Seren Global Media Limited | Information & communication | 2011 |
| Seren Lighting Ltd | Manufacturing | 2018 |
| Seren Property Group Limited | Real estate | 2021 |
| Seren Renewables Ltd | Energy | 2007 |
| Seren VET Group LLP | | 2022 |
| Seren Vets Carmarthen Ltd | Professional & scientific | 2022 |
| Serenity Care Swansea Limited | Health & social care | 2026 |
| Serenity Hair by Chloe Ltd | Other services | 2014 |
| Serenity Sunrise Ltd | Health & social care | 2026 |
| Serges Barbers Ltd | Other services | 2025 |
| Serges Turkish Barber Ltd | Other services | 2020 |
| Serious Games Ltd | Professional & scientific | 2025 |
| Sermo Media Limited | Information & communication | 2015 |
| Serodio Ltd | Extraterritorial | 2024 |
| Serv&Co Ltd | Wholesale & retail | 2024 |
| Serve Digital Ltd | Information & communication | 2026 |
| Service Assured (Swansea) Limited | Administrative & support | 2008 |
| Service Direct Limited | Health & social care | 2023 |
| Servmax Limited | Professional & scientific | 2019 |
| SES Estate Ltd | Real estate | 2026 |
| SET Estates Ltd | Real estate | 2022 |
| Set-Rite Construction Limited | Construction | 2016 |
| Seth Carroll Design Ltd | Professional & scientific | 2022 |
| Setlit Ltd | Information & communication | 2026 |
| Severn Bullion Store Limited | Wholesale & retail | 2025 |
| Severn Innovation Ltd | Professional & scientific | 2015 |
| Severn Systems Software Limited | Information & communication | 1985 |
| SEW & SEW Embroidery & Printing Ltd | Wholesale & retail | 2024 |
| Sewroc Limited Limited | Other services | 2020 |
| Sextons Cars Ltd | Wholesale & retail | 2017 |
| Sfca LAB Ltd | Wholesale & retail | 2025 |
| SG Evolve Ltd | Real estate | 2024 |
| SG Locksmiths Limited | Manufacturing | 2016 |
| SG Rail Safety Ltd. | Construction | 2016 |
| SG Renewable Solutions Ltd | Construction | 2023 |
| SGE Commercial Limited | Real estate | 1997 |
| Sgrs Holdings Ltd | Financial services | 2017 |
| SGS Couriers Ltd | Transport & logistics | 2021 |
| SGW Plumbing & Heating Limited | Construction | 2017 |
| SH Smith Consulting Limited | Professional & scientific | 2017 |
| Shade Nails & Beauty Ltd | Other services | 2020 |
| Shaheen Bibi Ltd | Other services | 2023 |
| Shahwetigs Limited | Health & social care | 2014 |
| Shahzain Ajmerwala Ltd | Information & communication | 2025 |
| Shak & Brothers Ltd | Hospitality | 2018 |
| Shake N Bake Ltd | Wholesale & retail | 2024 |
| Shalders Catering Ltd | Hospitality | 2025 |
| Shalom House | Health & social care | 2005 |
| Shamanic Activations Academy Ltd | Education | 2023 |
| Shanam Enterprises Ltd | Wholesale & retail | 2025 |
| Shane Davies Services Limited | Construction | 2018 |
| Shapetastic Software Limited | Information & communication | 2014 |